The safest option here is to create the two new VLANs and assign them to STP and relevant interfaces etc. as necessary. Then remove the Self IPs and any routing entries associated with them from the 5x VLANs and recreate with the 4x ones. This will involve downtime.
Alternatively you could effectively stop all LTM/TMM services and edit the configuration files manually changing all instances of 5x with 4x and then starting up again. However, if you miss anything the config will probably fail to load until you find and fix the issue.