You can't assume a milligram is a milligram. And you certainly can't add the dose of one medication to that of another and get a sum that is in any way useful. It's surprising that you waited to age 21 to know that.
The Robaxin dose is in, fact, quite a conservative one. 1000 or 1500 mg four times a day is a more common dose for Robaxin, and 600 mg four times a day is a common dose of ibuprofen. He's already reduced the dose, perhaps because of your size if you're small, or concomitant medical problems, or just plain conservatism to limit potential side effects.