I'm seeing other answers here which assume no change in armour design, I would argue that that is unlikely to be the case.
Disclaimer: I am not an expert in metal working or medieval armour, though I do have an interest in the area.
One answer argues that lighter metal does not help against blunt weaponry, but this is not necessarily the case. There are several ways armour designs can be changed to help absorb the impact of a heavy blow, but can only be implemented to a certain degree in traditional smithing because of the added weight. Areas of overlapping, but not touching, plates are one example. Each plate takes a certain amount of energy to move, and so consequently decelerates the incoming weapon.
If you're metal only weighs a quarter as much as steel, than you could have three layers of armour. These layers would be most effective if the hung slightly away from rather, or we interspersed with padding.
-The innermost is heavily fluted (the lines you see on Maximilian plate) holding the middle layer away from it (and strengthening the metal), and is the heaviest, this layer covers important areas such as the torso and head, as well as a fauld/tasset structure.
-The middle layer is lighter, a kind of maille with well placed scales or plates upon which small spokes are mounted, which sits on the fluting over the torso, but also covers the limbs.
-A third and outermost layer resembles the classic "knight in shining" armour, sitting atop fluting on the helm and the spokes on the middle layer elsewhere on the body.
There are a couple bonuses to this type of layered design. Assuming each of the three layers masses similar to traditional Maximilian style plate, it is lighter. Plate maille with padded armour underneath was very warm, but you could place staggered ventilation holes in each layer without compromising your protection. It would be considerably stronger than traditional armour, as it offers three times as much material between your meat and your opponents weapon.
It would not make the knight immune to blunt force trauma but it would certainly help, and other design choices could be made to improve the armour further. It would however be in essence three suites of armour, and made from a more expensive metal, so likely be very uncommon due to the price-tag.
As for any concerns of speed or agility, it likely wouldn't change an enormous amount, knights were trained to move in their armour and it's weight was not really much of a hinderence to them.