If solving with four freecells is too easy, why not try two or three?
FCPro and Championship FreeCell. The people at Championship FreeCell estimated that nearly all deals (about 99% judging from their first sample of 500 deals) can be solved with only three freecells, about 80 percent with two freecells, and perhaps 15 percent with one freecell (see section 5 for more precise statistics). Thomas Warfield’s solitaire compendium package Pretty Good Solitaire, an excellent Windows shareware program with over 600 solitaire games (including FreeCell), includes the Solitaire Wizard, a system which lets you define your own games by setting a handful of parameters. It is simple to use this to set up FreeCell or Baker’s Game with any number of freecells up to 8, and with variable column widths. I first saw these options in a shareware version of FreeCell for Windows 3.1, written in 1992 by Marc L. Allen. I expect it must still be available somewhere on the Internet, but I can’t give you a current URL.
