The 3rd Edition was my first manual dealing with cast bullets and it really helped me get started casting. I purchased the 4th as soon as it came out and was disappointed. The "how to" section and shooting/casting articles in the 4th are nowhere as helpful/interesting to a new caster as the 3rd. There may be more bullets and powders in the 4th, but unless you're into Black Powder Cartridge Competition shooting, the 3rd is much better and you don't need a Phd to read the metallurgy chapters...
If Lyman were to bring the 3rd back, I'd buy another one.