Slow start threshold = 18 MSS / 2 = 9 MSS
Congestion window size = 1 MSS
Window size at the start of 1st transmission = 1 MSS
Window size at the start of 2nd transmission = 2 MSS
Window size at the start of 3rd transmission = 4 MSS
Window size at the start of 4th transmission = 8 MSS
Window size at the start of 5th transmission = 9 MSS
Thus, after 4 successful transmissions, window size will be 9 MSS or 9 KB.