How is the JEE Mains counselling going to work after the 3rd round? I am pretty confused with the process. Can anyone describe it clearly?

Thanks for A2A!
Yes your seat will be discarded if you are not satisfied with your seat in 3rd Round and want to participate in 4th Round in which you can also modify your choices.
Spot Round is held for taking admission to SFTIs and allotment of remaining vacant seats in Govt. Institutes.
In Spot Round you have to pay Rs. 40000 advance which is non-refundable and report to allotted institute if you get admission there.