Installation of 4 channel Amp
 

Do you have to have 2 sets of RCA jacks on the back of your head unit to install a 4 channel amp?

no, most amps have line level input. in other words, the wires that normally go from your deck to your speakers, would go from your deck, to your amp, and then speakers.
high-voltage pre-outs are the best for good quality sound though

no you don't have to have two sets of rca's, if you have one, you can get a y-connector and split each side off to get four out of two. THe only problem with this is you wont have front, rear fade, it will just be left, right.

That is backwards. The high voltage outputs can feed speakers directly. The RCA pre-outs are low voltage (line level). Most amps can accept either, but it is preferable to use line level connections to the amp. (Unless you only have one set of line level pre-outs and want to maintain fader control.)

Most nice amps will duplicate the signal and have a switch for 1 or 2 sets of RCA's. Just make sure you get an amp with this feature and you won't need 2 pairs. LIke i said, most amps have this feature

You are using 2 channels for fronts, and bridging two channels for a sub?

Get a line level converter with 2 channels. Radio Shack sells one for $9.99. Walmart has a 4 channel for $17.99 (which obviates this fix). Hook this up to the speaker out on the head unit.

Run rca from the converter to the amp channel 1/2. loop from rca passthru 1/2 out back into amp in 3/4.

Hokey as hell, you'll have no fade control, but you won't have to rely on the amp providing you a single/double combine input switch. IF you need fade, use the 4 channel line leveler. If the amp has phase, that might be really useful for this solution.