There would be a second grade citizenship for somebody.
My first reaction to the scenario is that unless it is an occasional aberration between two short lifers, the elf people are going to become the dominate species on the planet. It will go from humans liking the new elves, but as the elves become more prominent in society, the humans will start to become more biased toward them, because the elves represent a threat. And the elves will start to become more bias towards the human, considering them something less then an elf.
Whatever it is that makes the elves live longer is in their genetics. Elves would tend to mate with elves since they would want their children to live as long as they did. Maybe not so much at first because there are not many elves to pick from and humans and elves still get along pretty good. However as time passed and more elves were around they would tend to mate with each other with more frequency.
My rudimentary understanding of how cells work, is that cells contain a clock work of sorts and a mechanism that limits the number of times a cell splits. When the mechanisms fail, we develop cancer. Essentially the cells reproduce erratically and grow into tumors and such. The elves would simply have this cancer like quality to their cells, but instead of the cells growing into unhealthy body mass they would be healthy cells that kept the elves healthy, young looking and long living. The traits that made this possible would need to be passed on from elf to elf with reproduction.
The elves would first segregate from the humans in small and large ways than they would dominate the humans and eventually the humans would become so marginalized that they would cease to exist as a society and eventually die out. It would be an evolutionary event, similar to others in human evolution were the once dominate humanoid just became a trace in our genes.
Two things are very strong factors in the elves coming to dominate just by virtue of their long life spans, Acquisition of wealth and power, and reproduction. Humans will simply not have the time the elves do to do these things. When resources become scarce, and they well, the elves will win the day and eventually the world. Human kind will have evolved in to elves, and the short lived will be no more.
Society would be very different then it is now. Marriage would be out, retirement would be different depending on the resources society has. It would be difficult for most of the people to acquire enough wealth to retire for a long period toward the ends of their lives.
This is an institution that will dramatically change. It will not be idealized as the lifetime commitment that it is now. It will be some kind of commitment that is relatively short compared to ones life span. It may not exist at all. Elves will engage for a few decades to a one night stand to reproduce. How long they might stay a family would just depend. But certainly with youthful urges not many are going to last for the 4-500 year duration of life. It maybe something you do many times over the course of a life. With the number of children possible to have and the number of decedents possible to be living during your time family is going to take on whole new dimension's that may or may not be important to a elf.
Retirement at the end of a long life like we know it now is simply not going to happen for most people that live 4-500 years. The genetic clock running out is not likely to be the cause of death for most people. Untimely deaths due to accidents and disease will kill many more people then old age. Planning for retirement may very well be considered an exercise in futility for most people.
It is extremely difficult to come up with a number for people that die accidentally. About 250,000 of 2.4 millions of Americans who die, die accidentally each year. Of everyone who dies in a developed country, 7 out of ten of those deaths are people over the age of 70. With that in mind we can say that accidents and disease account for about 30% of the death rate each year, or about 800,000 people die of the 300,000,000 in the US. If your multiply the one third of a percent that dies of unnatural causes each year you have roughly 3% of a population dead every decade, about 30% every century who die from accidents and disease. We could lower that figure by removing a lot of deaths that occur from heart attack, making an assumption that we would not tend to drop dead from heart attack so much because at fifty we would have the hearts of twenty year olds. And perhaps when you add war into the equation the rate might go up. If you consider super bugs that are making antibiotics less effective, then the figure might go up to plague like stats in some years. Over the course of 500 years a lot of things can happen that make it the exception to live to be a 500 year old.
This kind of begs the question, at what age does social security kick in?
Anyway back to retirement, you would not retire. At some point in your life you might become disabled, unable to work and there would be some program to take care of you.
For most people, depending on how mundane the job is, short retirements more akin to sabbaticals would be the norm. You might take a couple of years off to play. You may after fifty years in a job decide to change careers and go back to school for a decade then get a new job. You might sell a business and be able to cruise for a couple of decades, dabbling in shuffle board and basket weaving until the money ran out. You may just have a very good time with your career and never do much besides vacations.
The emphasize would shift from end of life medicine to quality of life medicine. Research would be mostly in accessing risk factors and mitigating those factors. Smoking would be abolished. Other risky behavior would also be looked on as much more serious. You would after all be risking a life that has the potential to last five hundred years as fairly healthy, rather then a life of 70 years that starts to degrade health wise the older you get. People would eat better to make sure their heart and other vital organs do not give out prematurely. Many drugs like ibuprofen for example, that have long term health risks like kidney damage would not be allowed. The elves have solved the biggest medial problem we face today, the genetic barrier to a long life. They would now focus on the other things that end life prematurely.
Law is going to go through changes as the ratio between elves and human changes. At some point as the group of elves grows larger and the fear of them becomes more real, the laws against them will start to resemble the racial laws of the south, the Nuremberg laws of pre WWII Germany and other status against groups in other countries. This still will not stop the eventual outcome of the elves becoming the dominant humanoids on the planet. At some point the balance will tip in favor of the elves, and they will do about the same to the humans as the humans had done to them.
This may not only be in legal matters it may be that this leads to war like conflicts at about the time the two powers are about equal. Still the elves will come out ahead, just because they can produce many more battle worthy citizens for soldiering then the humans can out of about the same size population. Any population that the elves loose during a war can be replaced much more robustly.
Crime and punishment for elves will be much different from today. The main problem being is locking someone up for life. Can they afford to keep someone in prison for five hundred years? Or would this mean that rehabilitation takes on a new urgency and meaning.
Term limits for judges and politicians would become the norm.
unfortunately, there will always be insurance salesman and adjusters to make sure your wasting your money.