Yep, that's exactly what I notice. I try to avoid going up to 6500, so I usually shift just a tad after 6000. in 3rd I do feel a slight dip in power like you said.
Actually having to rev it up to 6k to get a good pull out of 4th reminds me of the block brace spoon made for the honda civic. it makes the engine run smoother and bring its useable rpm range up to near 10k. you won't make much more power by reving it that high, but it helps since when you shift, you stay right inside the strongest point in the power band. but anyways, that's off topic...
3rd gear will top out a 90mph at around 6k I think, maybe a bit higher, so yeah, shifting into 4th wouldn't be much of any use. But because of that I sort of want to put a 3.70 into my 78 instead of the 3.545