As a practical matter, you should attempt to pay your rent regardless of the apparently improper notice in order to prevent an eviction. Unfortunately, without reading the terms of your lease, it is impossible to know if you are liable for rent for the full month of February or only until February 20.
Finally, although your security deposit can be applied to unpaid rent under Civil Code 1950.5, your landlord must refund the balance of your security deposit within 21 days of your move out. Further, your landlord may only deduct from your security deposit specific items like unpaid rent and repair of damage beyond normal wear and tear. Otherwise, you may have a claim for bad faith retention of your security deposit.
There are so many things wrong with this notice. As my esteemed colleagues have advised, a notice demanding rent must be in the form of the alternative (ie, pay "or" quit). Further. if the notice was given to you (and dated) 2/4/13, then it does not expire until the end of the day 2/7/13.
Again, try and comply with the bad notice to avoid further action. In the alternative, write the landlord (keep a copy) and let him know all the reasons the notice is bad. Offer to have him use your security for unpaid rent and see of you two can work out terms. Everything in writing! Be ready to comply if and when you are served a proper notice.
If the landlord files an unlawful detainer (attaching the 2/4 notice), your response should be in the form of a demurrer. If you are low-income, call or visit a local legal aid clinic for advice on its preparation; if not, you need an attorney (and that will cost a lot more than $625).