life yikes it would have to be under some extrodinary circumstances never heard of that i know a buddy of mine in mass got popped for three prtty good size plants in his backyard and got 1 year probation and a whole lot of grief
The UK law is pretty funny. But their serious charges start after 9 plants. They also treat you worse if you have previous criminal convictions, especially with cannabis cultivation.
For 4 plants and no previous convictions, spent or unspent, you'd probably have your plants/equipment seized and be let off with a warning and/or a fine, but that isn't 100% guaranteed.
For more plants and previous convictions, they'd try to throw the book at you.