A4000 has everything A1200 has but more. Ok, maybe the expansions are more expensive and it's doesn't have a clock port but you can get a clock port with an expansion card, the accelerators for the A4000 are better and a Zorro/PCI sound card is better anyway.
You'll still be able to play all your WHDLoad games but be able to do way more in terms of classic Zorro cards and RTG (unless you get a Mediator, which I personally don't like because it's a hack without DMA).
The problem you might end up getting is that after you tasted the A4000 and it's better expansion capabilities you'll probably want more and for that there is only AmigaOS4 or MorphOS machines available.
So maybe consider keeping the A1200 for classic stuff and an NG Amiga for the newer stuff.
