From what I've seen in job announcements, that seems to be the minimum that most tug and barge outfits want you to have. It's the company requirement, not the USCG's.
By regs, a DDE is good on anything 500 tons or less. Since most boats that size aren't required to carry Engineers most people I've met stick with the 4000 to avoid the electrical test that comes in with DDE Any HP.
I've heard through the grapevine that with a DDE 4000 you can pick up a gig pretty easy through 333 in NY but I don't know that for sure.