Due to the heavy corrosion it is very difficult to give definite attributions but - the top left and bottom right coins have markings similar to low value coins of the Western Han Dynasty, 2nd Century BC to 1st Century AD.Top right may also be Western Han 175 - 119 BC. Bottom left is too far gone to even offer a guess!
Much better examples of these coins can be purchased quite inexpensively ($5.- to $10.-) from reputable dealers listed on:-
http://www.vcoins.comI don't believe soaking them in olive oil will help.