Suzuki GZ125 trouble starting

[chuffzilla]

Bike has been running fine up to about two weeks ago,

Over the course of about a week it began to struggle starting, the engine would turn a couple of times but before firing (when it started up everything ran fine).

After a week it stopped starting, the engine would maybe turn once but after that it makes several quick clicking noises (from what I assume was the solenoid).

I gave the battery a charge up on a 4A charger (which seemed to charge quite quickly) stuck it back in and I got about 3-4 turns of the engine before the clicking started up again.

Decided to drain the battery completely and put it on a 2A trickle charger for about 8 hours (charger said it was done after about 4 hrs), tested the voltage which was reading about 13V, this dropped down to about 4V when starting (or attempting to anyway).

I have changed the oil and put in a new spark plug as the old one was a little oily (new one is a tiny bit oily when I took it out but seems OK)

Am at a bit of a loose end as to what to try next,
the battery is about 4 months old so I am not entirely sure it is the problem (but it did have some white deposits on the positive terminal even after cleaning everything up before putting the new one in so I am sill a bit suspicious)