I have a question on sizing multistage heatpumps.

My house has been figured for 3.5 Tons. It seems that the multistage heatpumps (Carrier, Trane, Lennox at least) only come in whole sizes now. Given this is a 4 Ton unit be acceptable or would you advise on only using a 3.5 Ton single stage?

The variable speed air handlers do come on the half sizes so would you mix a 3.5T AH with a 4T compressor (I suspect not) or would 4T AH and 4T 2 stage work for 3.5T with extra capacity for those really cold days?