1) Any owner can move into/onto or otherwise use land of which he is an owner, as long as he does not bar other owners from use/access.2) An owner cannot get "squatter's rights" against other owners (better referred to as "adverse possession") because his usage is permitted--he's not a "squatter" and his use/possession is not adverse to (i.e. not hostile to) the use of the others.3) If the 4 owners cannot agree as to the use or disposition of the land, one or more of them could bring a judicial action (a lawsuit) for "partition"--basically, for a court order chopping up the land or ordering its sale and the distribution of the proceeds thereof. This is a complex action, so it would be advised to retain an attorney to help you.