Is platform migration required between big-ip 4000 and i4600?

You absolutely need platform-migrate if you are importing to a new device, but afaik it will not map the old interface names to new ones, and there is another approach that may work for you:

Configuring the network manually on the i4600 with new IP addresses, upgrading old and new pairs to the same tmos version, then joining the i4600 to the old cluster, then kicking the 4000 out of the cluster, and finally upgrading i4600 to 15.1.2

This requires having the same license and modules provisioned

Just in case: K9476: The F5 hardware/software compatibility matrix