It looks like the 10x5.5" DD elliptical and the 5" round? DD come with the pro-pack. So it seems to me you have the small end covered. The 10x5.5 concentric seems like a duplication of what you have in a DD and will not do as well in mineralized ground. The 13x15 DD would be interesting if not for the fact that you have a 4500. For hunting gold with that size coil I'd rather use the 4500. I think I would just stay with what you have and put the 5" on and use the machine for fine gold like a Gold Bug.
Regarding your coil collection for the 4500 I completely understand. But I don't have enough so I got an idea. You could go through the collection and find the overlaps. Then you could post those to the classifieds and I could add them to my collection Hee-Hee-Hee - Just having a little fun. I just recently added the Detech 8" DD to my collection for use with my GP 3000. So far the test run in the park showed good stability and no falses on bumping. I have 2 16" NF round coils DD and Mono, a 14" NF EM, 12" NF EM and the stock 11" round DD. So this coil fit into that small space and I'll use it where I have hotter ground and hot rocks. Oh yeah it is also waterproof. Most guys would probably have bought the NF Sadie and I thought about that but since I don't have a GPX I thought the DD would work better with my 3000. Probably the only coil that is missing from the collection is a 16" elliptical but since I have two rounds in that size I hesitate.
HH