Why stop at four? I say ball up a fist and stick his whole forearm in there! I hear its amazing. Oh and it goes back to normal right after. You actually get tighter because it stretches the muscles and makes them rebuild stronger. Like working out..
I am not sure about exercising that very part! Bu I do know that once a woman gives birth, she gets really loose thereafter, my girl is hardly any tighter after many sex session, it used to be one finger, then 2 and now 3 for me! So it meant it gets looser as it stretched, as using one finger now meant nothing! So you think about it! I would want my penis to feel good, not so much my fingers! :)
First, although Vag penetration enhances the experience, the main event to get you off is up above at your clit. Two fingers, side by side should provide enough sensation at your Vag entrance to to do that and reach above to your G-spot behind your pubic bone. More fingers just get in the way, although the tight fit can be good at the opening. but not farther inside.
Your Vag is quite elastic (passes a baby's head) and will retract. But if you do this a lot, it will "learn" to open up when it is contacted and seem loose for intercourse. If you don't want that, don't "teach" your Vag with more than two fingers. If you do and then stop, it will "forget" after a while.
Your vagina will go back to its original size after playing around. It is made to stretch. You came out of your mother's vagina, as a 8 lb baby. That's an incredible little device that's attached to you, don't you think? A four fingers unless you guy has Shrek hands isn't going to damage your lady bits. If you feel comfortable when he's fingering you with that many, you have nothing to worry about.