Well, do it.
1 Apr 2002 Deposit 4000.75
Days to 30 Sep 2002 = N <-- You figure this out
I = Prt = 400.75*0.04*(N/365)
Add interest to all the other payments prior to 30 Sep, combine them all into one large value, and you are ready to calculate the interest to 30 March 2003. Then you are almost done.