if i get 4th choice in 1 st round of josaa councelling and 2 nd choice in second round than can i float my second choice to get my first choice in further round.

No, directly floating your choices to get a higher preference in subsequent JoSAA counselling rounds isn't guaranteed.

JoSAA uses a demand-filling process. In each round, it allocates seats based on candidate preferences, institute/branch choices, and candidate ranks.

If you don't get any of your higher choices (including your 1st choice) in a round, and a seat is available in one of your lower choices (like your 4th choice), JoSAA might allocate that seat to you.

However, there is no guarantee that your 1st choice will become available in later rounds.

During the second round (or any subsequent round), you'll have the option to either freeze your allocated seat (4th choice) or float it. Floating means you're willing to give up your current seat for a chance at a higher preference in the next round.

If you are happy with your allocated seat (4th choice) and don't mind attending that college/branch, you can lock it (freeze). Consider how likely it is for your 1st choice to become available in the next rounds. If the seats for your 1st choice are filling quickly, floating might be a gamble. You might end up with no seat at all in the next round. Floating involves some risk. You might lose your current seat (4th choice) and not get your 1st choice either. Locking secures your current seat but eliminates the possibility of getting your 1st choice.

I hope it helps!