Four year old son is constantly getting into my underwear, pantyhose, socks, bras, and fishnets. Help.?

There could be any number of benign reasons he likes playing with the items you describe and none of them have to do with sexual identity. They're soft, they're different, they carry associations of you, they're kept in a small space that feels like a cubby house, and playing with them gets him attention. I would put any of the items you don't want him playing with (e.g. stockings because they might tear) out of his reach and go to a department or fabric store and buy him some soft squares of material (like scarves). Put them within his reach and see what happens.