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In general, I recommend to use a dedicated comparator device as opposed to using an op amp as a comparator. One of the reasons to use an op amp as a comparator is that you may need a few channels of op amp and a few channels of comparator For example, you may purchase a quad op amp, and use two channels as a comparator and two as an op amp. If you plan on using all four channels as a comparator, why not just use a comparator device. To directly answer your question, yes, if you can use one channel as a comparator you can use all four as a comparator.
Here are some reasons not to use an op amp as a comparator.
Comparators are designed and optimized to act as comparators and op amps are not optimized for this feature.
Comparators have rail-to-rail common mode range. Some op amps do not have rail to rail output range. When in an undefined common mode range some op amps will have an undefined output. The LM324 has common mode range to 1.5V below the positive supply. I do not recommend not this device with signals in the undefined common mode range (1.5V below positive supply and above).
Some op amps do not allow a wide differential input signal because they have anti-parallel protection diodes across the input (see Op Amps used as Comparators—is it okay? for info). Note that LM324 does not have this issue, so it is ok.
Op amps have overload recovery time. This means that when an op amp drives into the power supply rail it takes time to get out of this state. When used as a comparator the op amp will always drive into the supply rail, so it will always experience this overload recovery time. Comparators are designed for this kind of operation and will not experience this recovery time and will transition much faster.
Ultimately, if you have been using LM324 as a comparator and are satisfied with it's operation, you can use all 4 amplifiers in a quad to do that. If you have not used LM324 as a comparator, I suggest you select a dedicated comparator device.
