Actually, you
can have 4, if you have a controller at the secordary floppy
 controller address as well as at the primaryfloppy controller address.
  Each controller can only have two drives because of a silly hardware setup
 chosen by IBM. 
 But soon after that, Hardwarepatches (and BIOS versions) where available
 to connect again 4 Drives to a PC with one controller (Or if you don't want
 to modify, buy a third party solution). I have still 4 FD drives on my
 PC-XT via one FD controller. 
 
Sanyo MBC-55x-2 series had a 4 floppy controller but DOS saw only A: and B: and
it's hacked DOS used it quite well. The bum end of it was that RAM disks and hard
drives went to E: by defualt. An aftermarket DOS (name escapes me now) made use
of them and even pushed the hard drive drive (if equipped) to C: and routed the
others around rather than trying the tricky "assign" command  to do this.That
was
the only real nice thing about that silver piece of human waste. Who would have
thought a PC level machine would have no ALT key?