1Hey I solved it and got the same result.
here the other three balls will be moving symmitrically about the x-xis making an angle of pi/4 with it so 3cos(pi/4)vf=vi am i thinking right nishant
The 
The slighly less colored ball hits the 3 balls which are touching each other.
The three balls at the back are at rest.
The fourth ball hits the three simultaneously....
Find the velocity of the 4 balls after colission!
I know after the Chemistry Question of the day (18th November) you "may" kill me for making you guys think so much in 3-D..
But still this is a physics collision problem... i hope it makes u feel at some ease than the last Chem Question of the day which no one could solve/understand !
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7 Answers
1velocity of the lighter ball = 0
the other three balls have equal velocities of = (v√2)/3
where v= velocity of the lighter ball
the three ballls travel at an angle of 120° to each other
this cancels out total momentum in the plane ( it was initially zero)
1
1actually vibhav i got that they will be travelling at an angle of 120° to each other
and the got the final velocity by working on a pyramid and applying some trigonometry!!
and some physics of course!!