Any person or association of persons severally qualified, as provided in section 71 of this title, who have opened and improved, or shall open and improve, any coal mine or mines upon the public lands, and shall be in actual possession of the same, shall be entitled to a preference right of entry, under section 71 of this title, of the mines so opened and improved: Provided, That when any association of not less than four persons, severally qualified as provided in section 71 of this title, shall have expended not less than $5,000 in working and improving any such mine or mines, such association may enter not exceeding six hundred and forty acres, including such mining improvements.