As you're seeing in the responses below, tread carefully. If you can't afford the year 4 payments today, and you're banking on your income going up to meet the projected payments, you're taking a big risk. If your income doesn't go up as planned, or if something happens to your job, you stand a good chance to lose your home and damage your credit for a number of years after that.
Here's an alternative to the 3-2-1 buydown. Tell the lender to utilize the money being offered to accomplish a 3-2-1 buydown, to buydown the rate for all 30 years. The rate will not be bought down as much, but it will be permanent (for all 30 years), thus your payment will stay consistent month in and month out for the term of the mortgage. This is a much safer route for you.
I hope that helps.