Thread concurrency is essentially the number of individual Scrypt processes that will be run at the same time on your hardware, this needs to be high enough to fully stress the core without overloading it.
Generally speaking for a AMD cards you want to use numbers that are between 4 and 5 times the number of shaders on the card, whilst also being a factor of 64. Some ranges I would suggest testing for the cards you are both using would be:
HD 5450 (80 Shaders): 320, 384, 448
HD 4850 (800 Shaders): 3200-4032 (try those two and some in-between, just remember to make them divisible by 64)
With the HD 5450 I would also suggest trying a lower worksize (64 or 128) as it only has a narrow memory bus width (64-bit vs 256bit on most cards). From what I've seen I would say the HD 5450 should be able to do around 15Khash/s depending on the exact model/clock (speed/ratio) and the HD 4850 possible in the 100-125 Khash/s region, although I've got a bit less knowledge of 4xxx series cards.
As a bit of additional reading, this
thread on Bitcointalk goes into a bit of depth on the issue.