Per the manual, Appendix F: MIDI Implementation, Control NRPN Data section, NRPN 4188 is supposed to be for Sequencer Play/Stop though I didn't see it sent. Or sending MIDI start/stop.
Glancing at MIDI NRPN data from pressing the A/B/C/D buttons, it looks like the following may work:
A 431
B 432
C 433
D 434
When a button is pressed, that one is sent first with a value of 1 then the other three NRPNs are sent with value 0. So, switching to D would probably be sending the following NRPNs:
nrpn 434 1
nrpn 431 0
nrpn 432 0
nrpn 433 0
In case they might be useful:
Seq Mode nrpn 422 0-2 Normal, Gated, Trigger
Seq Direction nrpn 423 0-3 Fwd, Rev, Fwd/Rev, Random
Slew nrpn 374 4-5 Off, On
I did notice a possible UI bug after changing the A/B/C/D button selection via NRPN where the previously selected button could not be selected unless you select another first including the one just selected via NRPN. Will need to test out further before submitting as an issue.
Hope that helps.