Is 4gb enough to adequately play Halo Reach (minus the glitched co-op and firefight?)?

Definitely! Actual memory for a game (i.e. save progress, etc) is a relatively small file (<1 gigabyte). Plus, you can always use a flash drive for extra memory. I personally use a 2 gig flash drive, so I can take my profile wherever I want. It takes up very little memory.