You don't multiply each modifier individually; add all the positives from enhancements (so, in your case, +20% × 4 = +80%) and multiply the power's cost by that factor. THEN total all the negatives from limitations (in your case, –0%) and multiply by 100% minus that number.
So, if I had a 50 point power with a +20% enhancement, another +20% enhancement, and a –10% limitation, the final cost would be 50 × 1.4 × .9 = 63 points (as opposed to 50 × 1.3 = 65 points under the normal system).