Back cutting is a very very good method of cutting very neat rebates or mouldings .
You must observe the safety aspects when doing so , more common sense than anything .
You MUST use a power feed .
It works very well if you need to cut finished rebates not using a rebate block . ie Safety block with hss cutters in .
( rebate spurs leave a cut line , not good on finished work )
Also it comes into it's own if you need to do stopped work . Feed normally then back cut to machine the other hand .
When setting up the power feed make sure there is enough pressure and the machine bed is clean .
You may experience juddering when the timber is being fed .
Stop the feed and check the wheels and bed .
It is a safe method but needs respect .
Regarding 4 wheels for the method , again I've moulded miles of timber using a 3 wheeled powerfeed and never thought a fourth wheel would make any difference .
Regarding feed speeds , there are many theories on what is best but experience dictates whats best .
ie , material , depth of cut , number of pieces , is it the cut going to be seen etc .
Listen to the blade cutting , if it 'sounds' wrong then adjust .
Hope that helps !