well there shouldnt be a 4" drain on your tub line so im going to asume that you are talking about the cover on the floor next to the tub, if so that would be the lid to a drum trap, and yes they are a pain in the ass, most of the time they are damaged when trying to take them off, try taking a hammer and chesel and taping around the edge to unscrew, or you may hve to cut it or drill it out, but thats ok you can get a new one from your local plumber.

I attacked a similar problem this weekend. Couldn't get the lid off my drum-trap. Actually, I had a plumber out earlier who said he'd need to cut out and replace the drum trap, and that the access-panel didn't give him enough room to do that work, so I was faced with knocking a larger hole in the wall, and to get it large enough might have meant tearing into a load-bearing wall. This plumbing is all near a corner where the two walls connect.

Last week, the tub drain totally stopped functioning. In hopes of avoiding all the agony of tearing into walls, I bailed out the tub, disconnected the drain between the tub and the trap--I could get to that part of the plumbing--and attempted to snake out the trap from the bottom. I did manage to get out a few good clumps of old, smelly, yecky hair. The drain then functioned slowely for a few gallons, but then it stopped again.

In a desparate move, and thoroughly frustrated by now, I dumped a bit over a gallon (two 2.5 quart bottles) of Liquid Plumber into the tub. To my absolute and utter amazement, that worked. The tub drains freely now. I've never in my life had liquid drain cleaner actually work for me. And previous to this I'd dumped an acid-based cleaner down the drain to no-effect, so having Liquid Plumber (which is a base) work was a bit of a shock, but a pleasent shock to be sure.

I relate this story as a way of suggesting that you at least try a liquid drain cleaner first. (Maybe you've done that already) It may not work, but it doesn't cost much, and maybe you'll get lucky like I did.

Oh, and I learned the hard way to wear rubber gloves when you snake a drain. I still have a bit of black gunk under my fingernails. It took a couple days to get my hands clean after that messy work.