With the lower voltage spread you probably can get away with only changing one. If you are going to change the amp hourage you will have to swap all 4 batteries so they are all the same amp hourage. While the lift will still run with different amp hourage batteries it will start to cause more issues pretty soon down the road and it could possibly kill all the batteries or even internal components depending on the spread of amperage.

The reason the one battery gets up to 6.02v when idling is because it stealing power from the other healthy batteries, that is what can cause the others to go bad. Typically a voltage spread of 0.05v or more between each battery indicates that the battery is bad. They should all be close in voltage especially after coming off a charge or near 100%.