some things to consider, not meant to scare you away...
-do you understand power ground, remote turn on leads
-fuses
-speakers
-rca's
--heres a rough idea of what your'e in for.......
if so, then i'd say go for it, first decide where youre gonna put it, some 4 channels are rather large, while others are small, so find a suitable location, then depending on the size of the amp, get suitable wire
>i recommend nothing smaller than 8 guage for decent size amps.
so after you decide where youre' gonna mount the amp, measure how much power wire (wire from your battery to your amp), youre gonna need. typically its around 10-15 feet or so depending..... anyways run the power wire from your battery to your amp, and dont forget to install a suitable fuse+holder, about 10"-20" from the battery. You should have the amp relatively close to some metal to use as a ground, i used the black, carpet covered metal piece that springs over the gap behind the backseats. be sure to sand any paint off so you get a clean connection. Normally grounds shouldn't be much longer than 3 feet or so. So you have your amp powered and grounded, now you need to run a remote wire ( or power antenna), typically blue from your Head Unit to your amp, this tells the amp when to turn on, a smaller guage wire is normal for this, probably around 18 guage. After you run the remote you need to get some RCA cables to run the signal from the HU to the amp, you might want to measure how far you'll need to run them from HU to AMP, and buy a pair accordingly, i believe mine were 14/16 feet long. Hook up the RCA's.
>Now you should have your Power, Ground, Remote, and RCA's.....now the fun part. (scared yet?)
You have a few choices now, you can run wire from your amp to your factory speaker wire in your dash, previously attached to your HU, or you can run new wire to each speaker in each door. For slightly more ease, i just ran new wire into each door, but thats up to you...
I'd say just run speaker wire up to the corresponding factory speaker wire, if you dont want to run new wire.
heres a crude diagram that ive used before:
some things to note, your head unit should now not be attached to your door speakers, your external 4ch amp will be powering them....
-(the middle paragraph where it says, "instead they should be going to your amp"--i'm talking about your factory speaker wires, they should be going to your 4ch amp. all righty then...so if I haven't scared you off have fun, and be careful, and most importantly if your'e not sure if you can do this, leave it to the professionals, better safe than sorry
good luck