Was it $90 for
just the RAM? If so, that's not very cheap. An 8GB DDR3 SO-DIMM is about $50 at
Newegg and
Amazon. However, there is one thing to consider -- the current memory setup and possible future expansions. In other words, if you've only got two memory slots, which is common on a lot of laptops, and they've populated that with 2x 4GB, then to get 16GB, you will have to replace
both sticks. If they're using 1x 8GB, then you should definitely put another stick in, because you won't see any benefit from the dual channel setup (it can issue commands to two different sets of RAM at the same time). The only issue with that is that you usually want to avoid different RAM specs, so you probably shouldn't just buy RAM all willy-nilly.
As for my thoughts on RAM amount, I use 32GB in my desktop and 16GB in my laptop. The main reason why I use so much is that I'm a tab fiend -- I've used over 8GB of memory with just Waterfox (64-bit Firefox variant) alone.

I definitely wouldn't do 4GB, and I think 8GB is perfectly acceptable.