see time taken for particle to go from mean to one extreme = 1/4 th of motion
so time taken = 0.3s to move frm 0 to +4 cm
time taken to move frm 0 to 2 cm :
y = asin(wt)
w=2pie/T = pie/0.6
y=2, a=4
on solvin u'll get t = 0.1
therefore time to move frm 0 to 2 = 0.1s
and time to move frm 0 to 4 = 0.3 s
time to move frm 2 to 4 = 0.3 - 0.1 = 0.2
time to go and come back = 0.2 x 2 = 0.4s

at least i think so