It’s just like other rounds in IPU, the only reason to include this after 3rd round is to give some seats to deserving students rather than selling(ya it is completely senseless) them in spot round. In this you could retain your 3rd round seat if not alloted any seat in sliding round, but there is a condition to enter in sliding round which is, you have to pay the academic fees of 40k to get into it, which will be managed in your total college fee.
Hope this helps. Thanks