While your speculation that development is more fluid at this age is well founded, it's still possible testing could be helpful, especially if you found your previous experience to be informative, however, if I were in your position, I would consider waiting until your younger child is at least six (and maybe even just a little older--eight is a good age), so he can be administered the WISC instead of the WPPSI. If he is in the same range as his older sibling, he is likely to hit the ceiling of the WPPSI, in which case the testing results won't be as helpful to you, since all they will tell you is what you already know (which is that he is very bright!). The WISC extended norms allow for substantially more range in the upper extreme than the WPPSI does.

In the meantime, you can continue observing him and the way he interacts with and explores the world, which may well provide you with the kind of information you seek.
...pronounced like the long vowel and first letter of the alphabet...