Josuetas,

I think a hardware frequency detector like the LM567 or a digital switched capacitor filter is the way to go.

To ask a PIC16F to do a DSP function is pushing it. Last time I did something like this I had to move up to the PIC18F family because it handles RAM better and it multiplies much faster. If you only wanted to detect 350hz out of much noise and voice then I would try it. To detect 4 frequencies at the same time is no going to happen totally in software. I think you should move up to a DSP chip if you need to do it in software. Even then 4 frequencies at one time is a project!