T4 is the best sounding arranger out there. And the one main upgrade needed would be a downgrade in pricing
I don't own a T4 but reading on the features, They can consider the following features
- vst capability
- touchscreen OS
- sound synthesis on board
- audio loops
- audio midi synchronisation capabilities

I'm sure a lot of T4 owners will kick me in the butt saying they don't need these features for an arranger. But then hey .... what to add in an already amazing instrument !!