Haha. Professional installers do it all the time buddy. No knowledge?...are you kidding me? You're the one that's trying to tell everybody that an upgrade to 6.5's is considered a major modification...lmao...people these days...
If you had 4" speakers you would have to set the cross-over frequency to about 250hz. If you had 6.5's you would be able to set it to 100hz. Subwoofers are usually set from 80-100hz...with 4" speakers don't you think there will be dead spots? 100hz and 250hz is going to sound a whole lot different...it doesn't matter if it's a 5" hole versus a 7" hole, it's not that big of a difference...cmon now Mr. Professional.
6.5's would be considered doing it right. If you have 4" speakers why install aftermarket 4" speakers and a sub because 4" speakers simply lack mid bass. The sub is going to cut off at 100hz and the 4" components will pick up at 250hz...doesn't that seem not too smooth. If you're gonna do something do it right? Professional? You're joking around right?