I've just picked up my FREE copy of the 4th Edition rules for Flames of War. Battlefront are giving away copies this weekend to anyone showing their old 3rd edition rules, so existing players don't have to buy the rules again. They did this when 3rd Edition came out and its good marketing because it keeps existing players happy...and who doesn't like receiving a free rule book!
In fact you get two books when you show your old one. One is the core rulebook and the other is a compilation of all the special rules, converted and amended where appropriate to fit the latest addition. On first glance there doesn't seem to be a huge change to the rules, aside from the fact that the rulebook is considerably slimmer than the previous edition. They have achieved this by stripping out a lot of the unnecessary 'fluff' and concentrating on streamlining and clarity over flashy presentation. The result is that the new book is just 105 pages compared to 293 previously. IMHO its about time, I dropped my 3rd edition rulebook on my foot once and I nearly broke my toes!
Seriously though, any attempt to simplify the rules can only be a good thing I think. I have never liked rules that are so complex that games become a battle between the players and the rulebook rather than between themselves.