Playing this gadget, 4 shows a responding hand that pretty much wants to play 6NT, but is interested in a suit slam if a fit is found. Because we have the room, we add in that 4 isn't quite 100% forcing to slam; opener, with a 4333 minimum bids 4NT, so we can play it there.
Opener's responses are 4-card suits at the 5-level and 5-card suits at the 6-level. These are good suits, not junk. A 1NT opener ignores suits worse than Q10xx; a 2NT opener needs more, perhaps KJxx. With some interest in a suit slam, but without a good enough suit, opener can pass the buck with 5NT. If responder then shows a suit at the 6-level, opener is expected to pass with any four-card support, but bid on without it. Responder's other rebids are similar suits; ones he's interested in playing at the 6-level. If logically partner has bypassed a 4-card holding in the suit, it's five cards long. Either partner can end exploration by bidding 6NT. The only time we can stop short of slam is if opener rebids 4NT and responder passes.