Replaced Front Speakers

So I replaced my front OEM (non-HK) speakers on my 2003 325i saloon. I replaced them with Kickers. The problem is, they are 4 ohm speakers and the OEM speakers were 8 ohm. Now, I can hear the tweeter and mid-range too well but my new 5.5" Kicker speakers are very underpowered (I realize that I probably should have bought component, but I didn't).

I currently have a double-din Sony but I'm buying the Dynavin ASAP.

Any ideas on how I can correct my issue here? Sound is way out of whack...perhaps adjust the levels on my factory amp?

call up parts express and explain your problem, a simple resistor might be a solution for you, im not sure what kind of resistor but people at parts express might help you out

if you sort it out let us know what you did