Despite all the "bashing" CapitalOne takes, I happen to be a very happy customer. They recently helped me merge Visa accounts into one, getting my Visa Platinum credit limit to $4,100 and killing an annual fee. (That was very helpful, plus I recently paid off my auto loan with Cap1 in full when I sold my car, and I never ever had any problems with them). They do, however, have the issue that everyone hates them for and that is their failure to properly report your true and correct credit limit (rumor is they plan to start reporting this infomation by the end of the year, but who knows...) To get around this with your recent check I would simply do the following: Write the check out in the amount of say $4,850 or even $4,900 and deposit it into your checking account. Then, in a couple days after it clears, make a full payment (minus leaving abount a $10.00 balance) and you will create an excellent trade line on your credit report AND you'll have an extra 5 K. or so utilization amount which will really help raise your FICO score!
I say utilize this account and you will end up with a smile on your face in a month or two when your credit score makes a big jump; go for it!