Here's what I've found on my B2910:
Slight pressure on the reverse pedal or slow reverse motion to engage 4WD. Any forward motion to disenage 4WD.
Shifting between ranges works best when the tractor is stopped on level ground - in fact it doesn't even require clutching - just push the lever to whichever range you want .... no grinding of gears, it's prefectly smooth.
If I'm not on level ground, meaning the tractor's weight is pushing it forward or backward, then it requires some fudging - like a little pressure on either the forward or reverse pedals, clutching, stepping on the brake, etc.
I recently discovered that you could change ranges without the clutch - usually this works better than anything else.