I have heard of people putting 3.5" speakers there, but I havent seen 4" speakers done before. Your biggest problem is going to be depth, there isnt much there, the cooling ducts are right below the opening. I dont feel there is any reason for this anyway, putting speakers everywhere they fit will not equal good sound. Stock locations in volvo's are not ideal, they arent even very good, but they will get the job done. If you go to my website at Sound Domain, you can see what I did, but I will imagine that is more intensive than you want. I have kickpods, they sound wonderful, but they arent practical. I have tried everything in my car, infact my new project is going to use special doorpods and component speakers mounted in a sort of coincidental fashion. I have tried something like this before, and with only so so results, but I want my foot room back, so Im gonna try it again. Anyway, I recomend doing what Volvo did in the first place, get a set of Component speakers, mount the Midbass in the doors and tweeters in the dash. That will give you fine results.