avinash, one way to look at it is that you want to pick a random number from 1 to 4, or 0 to 3, if you want to be zero-based <img src="smileys/smiley1.gif" border="0" align="middle" />
Store this value in a local variable, and then use 4 sub-events to create the appropriate object based on the random number.
For example, if the value is 1, create object A. If value is 2, create object B, etc.
This gives you the flexibility of being able to do other object-specific logic at the same time, in case you needed to run an extra step whenever you spawned object 4 for example.
Hope this helps!