Well, a nanny is a heck of a lot easier logistically w/ 4 kids.
*However* I would suggest that your cost estimate is highly optomistic. Have you actually priced nannys in your area. I am in the DC area and the going rate is $15 per hour. For 4 children you might expect to pay closer to $20 per hour. Assuming you pay legally you can expect an additional cost of 10% for payroll taxes. Also legal pay often means you have to pay the nanny more per hour ($15 is the 'under teh table' going rate here).
If you need someone to drive consider that you may have to provide a car and insurance. You'll also need to pay during your vacation days and such.
Sorry to be such a downer. I've had a nanny the past 2 years nad we are letting her go at the end of the summer because the cost is too great. I went with an 'in home' daycare. However, for 4 kids a nanny may be the same cost as daycare. In which case, I'd go w/ the nanny. That way you don't have to worry about sick days (other than for the nanny). Nannys can also run errands, take the kids to appointments, etc. Be sure to run a background check and interview throughly, and call references.