Nope, you can't use it for main protective bonding as that often needs to be 10mm depending on supply type and connected directly to the MET, you also wouldn't expect to find a MPEB conductor in the CPC rail designated for a circuit.
I don't think I mentioned MPEBs tho?
Of course, your MPEBs should be checked that they are adequate before any alterations or additions are carried out (reg 131.8)
You would need to run the 4mm from the consumer unit all the way to each class 1 accessory in turn keeping it close to the original cable run.
In other words, in 999/1000 instances it would be just as easy to replace the old cable with twin and earth instead of faffing about with 4mm.
I don't think I mentioned MPEBs tho?
Of course, your MPEBs should be checked that they are adequate before any alterations or additions are carried out (reg 131.8)
You would need to run the 4mm from the consumer unit all the way to each class 1 accessory in turn keeping it close to the original cable run.
In other words, in 999/1000 instances it would be just as easy to replace the old cable with twin and earth instead of faffing about with 4mm.