Michael,
I used a new 5000H in NZ last week. The reel is essentially the same size in capacity as a stella 10000. It's very nice - really smooth, drag is good, a bit lighter than a stella 10000XG
When I saw your question I tried spool swops to see what works.
Looks to me you could run the old 4500 spools on the new 5000 reel. However, doesn't work the other way, the new 5000 spool is too big for the old 4500 reel. I can't comment on the new 4500 spool though as I haven't used one. Suggest you ring Brad Sissins at Daiwa who should be able to let you know.