Warhammer is not strictly part of the recognised, Chaosium based D100 family, but it does share a basic d100, roll under to succeed mechanic. There is little separating the core ideas of these games, but the character generation systems, combat, character characteristics etc. are very different.
1st and 4th editions both have their upsides, but neither is perfect.
WHRPG 1e is a favourite and a good solid, dark themed game. Switching to the more up to date Zweihander clone is always a temptation, but at 700 odd pages with several other massive tomes, it for me is too much of an ask. The 1e game is a full experience loaded with all of the classic feel of the original, (actual) warts and all.
WHRPG 4e is a vexing beast. I have had my enthusiasm for the game tempered by both my own experiences and the opinions of others. I should know better (and do), but still, the new comer is a little way from perfect. It has the right feel, the right goals, but needs some taming and clarifying. This is a prime example of a “perfect” glossy work of art that is not perfect after all, but defies the “fix on the run” feel of less ambitious systems.
I intend to use 4e as my “Dark and Gritty” game, hopefully putting together a group of players for the Enemy Within campaign when I have it all, but some things will have to change. I am dropping the advantage mechanic from combat, intend to use the simple SL system (or my own) and make some house rule sheets for things that will complicate life, such as shields etc.
It would not be a work of ages to convert the specific feel and themes of Warhammer into a standard d100 game like Legend or BRP, but the opposite is probably not true.