u/Acceptable_Bend_9578

Hello everyone i'm looking for some advice, I recently moved to the UK from NZ and signed onto a lease in March so it has been less than four months. I was just served by email (4 pm 30/04) a Section 21 to vacate my flat by 31st Aug (when original lease ended), however due to job seeking being terrible I was thinking of moving out late june/early july and finding a replacement tenant as my lease has a replacement clause. Am I able to do this with the s21? Is the s21 still valid if i've been on the lease less than four months (been going through Shelter, reddit, and UK rental laws). I haven't responded to the s21 yet as i'm not sure of my options.

thank you