It's difficult to answer this question, as we really need to know then intended outcome to comment on the process.
Looking at some basic recipes for mead, starting Original Gravities (OG) can be anywhere around 1.080 to 1.100. Of course it depends on whether your aiming for a dry mead or a sweet mead. You might want to go lower or higher. Of course the final AbV depends on your yeast and fermentation process too.
Given the lack of space in fermentation vessels, the best fermentable-per-volume is a dry sugar. Whether this will poorly effect the taste of your mead is up to you. Personally I would not go this way.
If you just want to boost the AbV you could add vodka, or some other neutral spirit to the finished mead. This will of course dilute the flavour too.
Personally, I would just drink the mead at 4% (I assume from the final gravity it will be quite dry), and plan better for the next batch. If this idea is not to your liking, and your question suggests this - then perhaps ferment a second batch of "strong" neutrally-flavoured mead, and combine this will the low-gravity dry meads to balance them out.
Recipes and articles on basic mead:
byo.com ; beerandwinejournal.com ; beersmith.com