Hatch cover missing - odd size? please help

I bought a boat that has 2 hatches installed (to fix the softspots)

One has a cover that is missing. Looking in the hobie cataloge I see they show a 4" and a 5". I measured my cover and it's 4.5 inches?

could I have a non-hobie hatch? Are there other makers/sizes?
Or is this one of those instances where they call it one size and it actually is another? (more common than I'd like to see)

Anyone out there have the hobie hatch cover that they can measure?
I want to order a cap - but not sure what to get.

the deck is curved, hobie offers a couple curved hatches. i had a 5 1/2" hole so i went with the beckson deck plates. they aren't curved but will still bend/screw in and don't look odd.

Maybe I wasn't clear. I have a hatch installed. But the screw in (white) cover is missing. I want to order a replacement. But when I measure the threaded circle cover... it's 4.5 (4 and 1/2 inches) in diameter.

Hobies catalog states theirs are 4" or 5" ? is that the entire thing or just the cover?
IF someone has a hobie cover, could you please measure your "cap" only? the screws into the circular flange?

Rojoyinc-I had a hatch installed by a Hobie Dealer in San Diego about20 yrs. ago. The hatch, which is now part of the deck, is 6¾"diameter; the threaded part is 5" diameter. The bag is a Fat Bag. The cap is 5½" diameter; the threaded part of the cap is 5" diameter.
Since there are so many choices of hatches and hatch cover kits, I really think you should talk to a Hobie Dealer. It's frustrating ordering the wrong part and having to send it back....
[email protected] (831)475-7245 seems very helpful and knowledgable.
Good Luck!