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

Thank you Amine. I ended up doing the procedure below, and it worked just fine. I am not to be held responsible for any typos or if this won't work for anyone else. For future reference:

Start conditions:
S1 - Source device 1 - version 12.1.0
S2 - Source device 2 - version 12.1.0
T1 - Target device 1 - version 15.1.2
T2 - Target device 2 - version 15.1.2
 
S1 is active, S2 is passive, T1+T2 are standalone
 
Before migration:
S1+S2+T1+T2: Create UCS to have pre-migration baseline configs (easy to understand names) - download all UCS files locally to management PC
 
During migration window:
S1+T1+T2: Change master key:  modify sys crypto master-key prompt-for-password
S1+T1+T2: Check master key: show sys crypto master-key
S1+T1+T2: Save config: save sys config
S1+S2: Make sure S1 config is copied to S2, confirm master-keys on S1 and S2 are identical "show sys crypto master-key"
 
S1+S2:  Create migration UCS-files that we will upload to T1+T2
 
1)  S1: Force offline (Device Management - Devices)
2)  S2: See that S2 is active
3)  S1: Shut down S1
4)  T1: Change management IP hostname to S1 (Device - Platform or config in CLI)
5)  T1: Upload the migration-UCS from S1
6)  T1: load /sys ucs <filename> no-license platform-migrate
7)  T1: configure interfaces, trunks, vlans
8)  S2: Force offline -> T1 should become online
9)  S2: Shutdown S2
10) T2: Change management IP hostname to S2 (Device - Platform or config in CLI)
11) T2: Upload the migration-UCS from S2
12) T2: load /sys ucs <filename> no-license platform-migrate
13) T2: configure interfaces, trunks, vlans
14) T1+T2: Sync device groups
15) Done