If I am not mistaken (it's what the G70 does, anyway) there are an independent number of Keyboard Parts to the Style and Song Parts. On the G70, there are eight Keyboard side Parts. UPR1 2 3, LWR 1 2, M/Bass, Melody Intell and D-Beam. Plus NTA channel, Basic Channel and a V-Link channel
So, as the engine for both sides is likely to be the same, there are SIXTEEN available channels for SMF playback (or eight for Style Parts).
I imagine the T4 is comparable. So, this leads me to think that there OUGHT to be at least FIVE untasked MIDI channels on the Keyboard side. Why can't THESE be tasked to a secondary Manual input, with split and layer capabilities like the main manual? You'd need an additional Page for the Lower Manual Mixer (same code as the main manual, so no biggie), and hooks to the Sustain and Swell pedal pages (again, no biggie), and there you are!
A totally independent lower manual, with very little extra work! And ALL of its parameters stored in the Registration for one button call up (maybe a 'Hold Lower Manual' would be a handy addition to the OS?), with up to five (and maybe more) Parts assignable to it.
I don't know about you, but what T4 owner WOULDN'T like this capability, if only for future expansion? Come on, Yamaha! It's only CODE...