Some are some aren't. They will almost always say if they are on the package, and often on the amp. Sub amps are normally bridgable, where 4 channel amps intended for speakers generally aren't.
A 4 channel amp would work fine with a sub, just bridge 2 of the channels and flip the LPF switch.
There are a couple of routes you could take, all depends on what else you want to include in your system. If you want to power a sub and a set of components for example, you could:
- Buy a 4 channel amp, bridge 2 of the channels for the sub and use the remaining 2 channels for the components.
- Or, buy a mono amp for the sub and power the components with a separate 2 channel amp, etc.