Correct tool to remove Uniglide freehub?
The freehub attaches to the shell with a big hollow bolt you access by poking a 10mm allen into the drive side. You can transplant an HG freehub body onto it, ideally one of the same or similar length (6, 7, and 8-speed UG existed) and also a matching cone and other axle hardware for the drive side so that the sealing works right. The upside here is a replacement cassette will be much more obtainable when the time comes. Wheel dish should be checked and corrected when you do this.
But, your question: to do a simple de-gumming type clean, the expedient thing is to take the freehub body off the hub, gently pry up the rubber seal on the back and take note of its direction, then run through whatever thoroughness level you deem necessary in of solvent/air/oil. Usually just working in something like Tri-flow or other light oil with penetrating qualities is good enough.
The 4mm splines take the Shimano TL-FH40 freehub body disassembly tool. This is one of the most fantastically seldom-used tools in all of bike wrenching. Full disassembly of the freehub is almost never necessary to get them working, and is very time-consuming because of the large number of teeny bearings inside. But, it is the only way to adjust acquired play out of a Shimano freehub body, which is done by removing one or more of the micro shim washers inside, which are there for that purpose.
The race that the FH40 engages with is reverse threaded to resist bearing precession loosening it. It's got things spinning against it on both sides so how that works is way above my head.
Edit: If this is 7400-level Dura-Ace, the freehub body attaches with a TL-FH10 and is only interchangeable with other similar Dura-Ace Uniglide freehub bodies. The 7400 hubs are the only Uniglide models with this caveat, to my awareness. Some hack replacements with more common units have been done, but I don't have anything to report about that.