frustratingly slow box at 4GB, but not 1GB of memory

[Todd Lewis]

Did you try compiling the kernel with the option

options            PAE

Your at the 4 gig marker. It wouldn't supprise me if your system is having
dificulty accessing the 4gigs. My rule of thumb is 1 gig of memory for every
256k of on chip cache. P4 have 256k~512k 1~2g memory. Xenons have 512k, 2g
for each processor. Xenon MP 512k to 1meg cache etc. etc. 
  
http://www.freebsd.org/doc/en_US.ISO8859-1/books/handbook/kernelconfig-config.html