^^^^^^
Good. As others have already stated, it would depend very much on soil conditions, tiller depth and tractor speed as to whether or not a 4' tiller would work for you.
Certainly, you want to avoid a situation where you are working the tractor or implement right to its ragged edge of capability.
When choosing a tiller consider the drive system (chain driven or gear driven), number of tines and shape of tines. There are tillers availabe that have the tines arranged in a 'helical' fashion on the drum. This feature means that not all of the tines are engaging the ground at one time. So fewer PTO hp is required per foot of width for the tiller.
Obviously 'virgin' soil (or any type) will be the most difficult to break up. In some cases.....plowing/discing first is a better solution than just attacking it with a tiller.
My tractor produces about 20 PTO hp. I run a 4' offset tiller on it. In virgin soil (sandy loam with significant grass growth) I can set the tiller to 4" of depth and run in low range, 3rd gear (roughly 1.4 mph).
It does a decent job in one pass without straining the tractor. But a second pass is still necessary to further chop up the grass and clods of soil. Any deeper than 3-4" on the first pass and the tractor quickly gets "all it wants".
So do consider all the factors.
Single pass, 4' tiller, 20 PTO hp, Sandy Loam: