I don´t have experiences with the ATB, but I have the ATC2, and it´s a great unit. I love the EQs, the compressors are fine, and the preamps very good, too. Not a lot of color/character, more on the clean an neutral side, but that don´t has to be a bad thing. And I´m quite sure the pres in the ATB are not better.
Anyway, I guess only you can answer your question, because the main question is, what do you need? Do you need more compressors? And more of the same typ, or would it be easier to ge something cheap from FMR for example for a differend flavor and more flexibility? Do you need the master section in the ATB? Do you allready have a monitor controller? Do you need the insert points, the sends, the busses? Do you allready have a summing box and/or do you want to sum OTB - the ATB4 give you like 12 (?) ins you could use for summing, each channel can be uses twice plus you have ins on the busses. Or would you use the ATB just as 4 channel strips anyway?
Only you could answer that question. If you just want 4 channels, I would go with the ATC, if some or all of the other features of the ATB are relevant for you, I would get the ATB.