I am filling the form 8843. During summer I was out of usa. So I want to know how to fill question number 4a and 4b? Should I put the number of days I was present in USA in 4a and the number of days I was out of usa in 4b?

4a is actual number of days you were present in US.
4b is number of days in 2010 you claim you can exclude for purposes of the substantial presence test.
If you are on F-1 or J-1 exempt period through the year, 4b is same as 4a.