I have an Alpine pdx 4.100 running 4 speakers at 100rms each, I was wondering if I can bridge the amp and still run all four speakers at 200rms each while still having a 4ohm load? (note:the pdx is rated at 2x200rms at 40hms when bridged)

Depends on your speakers and their impedance.

That said, it's pretty pointless to do it, as although you are running the amp at 2x200wrms, if you have 2 speakers per channel it each will see half the power (assuming they are the same impedance/ohms), but you will lose fading and full gain/filter control.

In short, don't bother.

As Louie points out, this arrangement will see the loss of ability to fade front to rear but if you already have it set equal front to back this matters little. Similarly, if they require an equivalent HP filter point wiring it as you'd possibly prefer is fine as well. The real clincher as to why you would NOT wire it bridged is because there will be absolutelt ZERO gain in power output from the amplifier. This amplifier makes 100WRMS x 4 or 200WRMS x 2 almost regardless of impedance load, so the net effect (again as Louie points out) is that each speaker will get 100WRMS no matter what you do, so the bridging method is a complete waste of time and arrives at a lesser degree of control and flexibility.