I've not actually tried TC4 yet.
How different is it? I know research is done differently, golems are controlled differently and there is some form of physical taint back, but otherwise? Have most of the items etc changed?
If its just the above I'd almost suggest just leaving it as is and listing the versions on an items page.
If there are more changes than that, separate version pages would certainly be sensible.
Someone did something similar on the buildcraft wiki, except they left the old version as the main page and the new version pages had /BC3. This was a bad idea as there weren't actually many changes, and the old versions continued to get edited more than the "new" ones.