Two:

1680x525 and 1680x525 or

Turn your monitor on it's side (if it supports that) and use 1050x840 and 1050x840.

Three:

One at 1120x1050 and two at 560x525 or

One at 1680x700 and two at 840x350.

Four:

Four at 840x525 or

One at 1260x1050 and three at 420x350 or

One at 1680x768 and three at 560x256.