A legacy drone (A4) is one that was built and sold before a certain date. This cut-off date keeps shifting. Originally it was January 1st 2023, at the moment it is January 1st 2024, which is when the new pan-European legislation package starts to be enforced.
Legacy drones are those which cannot easily be retro-fitted with the behaviour inhibitors demanded by Brussels.
An A4 legacy drone will be the most heavily restricted, not by firmware constraints, or hardware design, but by law.
The UK isn't dancing to QUITE the same tune as the European Union, but the CAA has adopted pretty much all of the EASA regulatory package, so older drones incapable of being hobbled are also going to face the greatest degree of legal restriction.
"....and how will the CAA know if the drone you fly doesn't fit in with the Law?" is a question someone is bound to ask. The CAA will know because you registered the fact with them when you applied for your OP and FLY number, which, if this occurred before Jan. 1st 2024, means at least one of the drones you own is definitely non-compliant.
They will also know because electronic conspicuity is already built into all DJI drones using GO4 and FLY in the form of DJI-specific (Aeroscope) UUID and RID.