You can have up to 2 parity disks. The parity disks don't have to be the same size as each other. Each parity disk must be at least as large as the largest single data disk.
So, you can replace one of your parity disks with the new 4TB and keep the other 3TB parity disk, then use the old 3TB parity disk you replaced as a data disk in a new slot. In that case, all your data disks would still be only 3TB, so each of your parity disk must be at least 3TB. One of the parity could be 4TB and the other parity could be 3TB since one parity disk can be a different size than the other parity disk.
If you did want to go to single 4TB parity, then both of the old parity disks could be reused for data.
Tell us how you want to proceed. Since you are going to rebuild parity in any case, if you New Config you could add the data disk(s) at the same time and not have to wait for them to clear.