140 ft is on the hairy edge of 6AWG for 60AMP. I think the Lowe's guy is giving you good advice, besides 4AWG THHN/THWN is only a few cents more per foot than 6AWG. If you go with 4AWG, be thinking 1 1/4 inch conduit to make the cable pull easier. No reason you could not put all the wire in a single larger conduit unless they are going off in significantly different directions. IE: all wire in one large conduit to the tub area and use a large weather tight box to direct the individual wires out to the final locations using smaller rigid or flex conduit.