Smart people who have money would turn it into goods that will hold value before the giveaway occurred. Gold is the classic. In this scenario, basic provisions would be important. They will also concentrate on certain investments. This hoarding would get the inflation started.
Some number of people would quit their jobs. After all, they're millionaires now. Besides, why bother working if you're only getting paid, \$400 a week but have \$4 million? So even if stores are still only charging the lower prices, you won't be able to buy. No employees.
Owner-operated stores will adjust their prices to make it worth opening. Prices will skyrocket initially, but after a few weeks, I suspect that it will settle down. Some people will have blown their entire windfall and have to go back to work. Wages will have to adjust to fit the new prices.
Note that the total M4 money supply at the moment is roughly \$30 trillion. This would add roughly \$1.3 quadrillion to that. So a 4300% increase. Once prices settle, expect everything to cost forty times as much. In the near term, this will likely spike higher due to the labor shortage.
After a few months, everything would continue with new prices forty or more times as high as the old. Most people who were poor before this will be poor afterwards. Some rich people will be a lot richer. Others will be impoverished. Some middle class people may do exceptionally well. For example, owner-operated farms will be in good shape. Well, at least as long as they don't get destroyed in the food riots.
Realize that in the short term, with no employees, grocery supply chains won't be able to deliver food. Some people will offer to deliver food at gouging prices. They may well find themselves in danger from the starving. Particularly since the starving may find paying the 4300% premium to be gouging, even though it isn't. \$200 for a bottle of milk may be expensive, but how else do you pay a minimum wage of $400 an hour? Food will rot in fields unless foreigners provide labor to pick it and process it.
What about the government? Realize that they also are now underpaying their workers. Aid agencies may be begging people to come back. Perhaps the National Guard and military will be more responsive.
There will be a wave of corporate bankruptcies as companies can't fulfill their contracts (no employees). Pensions will be permanently impacted.
Our currency would become more like the Japanese Yen. No cents, just dollars. The top bill would increase in size. Perhaps a thousand dollar bill. Smaller denominations would become coins. I'd put the switch at everything \$20 and smaller would be a coin. A can of soda pop from a vending machine might take a $50 bill to purchase. Or more.