Hmm I dunno about a drop-in giving you 4hp...i would say maybe just a tad bit more but not that much...it might help your fuel economy though cuz of the leaner mixture...
Uhmm I would suggest looking at
JWT
www.jimwolftechnology.com
or
Stillen
www.stillen.com
I believe JWT Makes the POP for the P11 models, im not sure though I have a P10. I would say this would give you the 4hp you are looking for. You will be able to move all of the restrictive airflow from the beginning of the airbox to the MAF.