Actually if you change the housing to a T4 On-Center, you also have to change the turbine wheel to a T4 wheel. Also, if you've already constructed the rest of your kit, then it will also necessitate a change to your piping as the turbo will be relocated a small amount. Even swapping to a better matched T3 housing might change it enough that you would need to redo your piping. Of course, if you haven't started that portion of the work yet, then you should be fine.
That 4 bolt T31 is not what I am referring to. The only turbine that comes in an on-center configuration is the T4.
There are couple pictures on this page:
http://64.225.76.178/catalog/inst_dimensions.html
In the right had side of the first diagram, you can clearly see where the foot of the turbine is under the turbo on the On-center housing and offset on the Tangential turbine.