The 48 months assumes you did 18 months as D/H or O/S to get an Apprentice Mate Tow endorsement. Then you did 12 months as App Mate to get the Mate Tow (plus the TOAR) then you did 18 more months as Mate to get the MOT, for a total of 48 months. To be legal service requires service on a tug over 26 feet long, tonnage is not a factor in a towing license.
Given that the USCG will soon change (massively increase) the service/training/assessment requirements for all STCW impacted credentials, you should also look into getting some Master in addition to MOT at your next upgrade or at least bumping the Mate 500 to 1600. Once you can document a year of service as a Mate Towing, you qualify for a Master 500 GRT Oceans endorsement. The Master 500 plus your TOAR and 30 days towing service allows you to sail as Master Towing on a tug up to 500 GRT. You should also apply for an endorsement as Master/Mate OSV. That would give you a 500 GRT/3000 GT OSV license. It would then allow you to serve on a special OSV,an AHTS (Anchor Handling/Towing/Supply Vessel) since you also hold the towing endorsement. The more tonnage and endorsements, the more employment options!
How much sea service and how much Mate service can you document right now?