With physical media, there won't be many issues. But the 4gb is ridiculously inadequate. I have more than 14gb of DLC alone and I rarely ever buy that junk.
Unless you don't mind deleting and redownloading DLC and patches to older games, you should upgrade right away.
I don't even know why they offer a 4gb console... even my watch holds 20gb. Least it was free though, right?