I have taken pictures in there. They allow flash photography, but I think a 4x5 would be difficult unless you went Weegee on them. There isn't much room for a tripod in most of it, though there are some spots where you could set it up. You would need to call ahead and ask permission if you needed more than a minute or so for your shot, and if you wanted to photograph outside the context of a tour. Note that they have the cave lit, so if you aren't using flash and you are using color you are at their mercy.

Everytime I've been there they've asked that the group stay together, so it was difficult to spend time photographing. Your best bet is to see if they will let you go down by yourself, but I bet that their insurance won't let them do that. But, it can't hurt to ask. Make sure you tell them you aren't going to sell the pictures for money (or not as is the case.)

Good luck,

Will