Can I just replace the OEM amp to afterMarket 4/ch amp? Will there be significant improvement over stock BOSE amp?

 

Only if you change out the stock speakers, other wise you will just be getting same crappy sound, but louder crap

 

Getting even a relatively (under $200) set of components (Kenwood comes to mind) makes a huge difference in sound. More so than if you change the AMP. I would recommend changing front speakers first, then go from there. 4 ohm speakers will work fine (but will be a little quieter at the same volumes than Bose so may have to fade to the front to balance).

 

With the current Bose systems, you'll want new speakers and an amp. Then consider getting an aftermarket HU. If you have the money, do it all at once so you won't have to take apart your car multiple times. I'm still working on my car and it's been almost a month. You can really hear all the rattles when you drive around in a stripped car but I'm still waiting on my parts to arrive.