Ooh...I hope the 4D doesn't make me regret my 5Dm3 purchase.
Well it would depend. Looking at the 1Ds3 vs. 5D2, the 5D2 had much better high ISO performance. A 4D body would likely be high MP, lower ISO range, and most likely poorer AF performance than the 5D3. If you do primarily studio work and no action/sports, yeah I could see a high MP, low ISO camera being "better." Again that would be a fairly specialized camera, which the 1Ds3 really was, more or less.