There are probably some threads around here and G3 that address this, but the differences are the kind of argument that ends up being a waste of time for everyone involved (c.f. "I call x a fix" vs "I call x a tweak"). Tutu v4 allows you to add things to BG and/or BG2, at the cost of potentially having a messed up game. That being said, some people don't think Gaider's Baldurdash fixes (mostly dialog) are fixes, and they are included in EasyTutu, so v4 still has a non-English and "purist" usage. It also used to be the only way to access The Vault, a formerly pre-Tutu non-weidu mod that did some cool things... But had critical errors that made it tough to complete. The Vault and most other strongly liked mods have now been converted to WeiDu, though I am not sure what The Vault's release status is.
Using v4, you will lose the Journal entries, which no one else successfully managed to fully accomplish, though Andyr had a valiant effort; rebuild of spells into the BG2 system so that they fully work (again, no one else has a good replacement), and almost all of Dudleyville and Baldurdash. In return, you get a choice of whether or not to install Baldurdash GTU, you can pick and choose Dudleyville options, and you can incorporate ancient non-weidu (and weidu - but- completely - override) mods before the conversion.