From no4, the route you decide to take to here is up to you. There are quite a few options - the most straightforward of which would be to follow the fence by the ruin southwards through the heather, and then turning sharply ninety degrees to your right to follow the old stone wall. This should take you to within sight of the gate, from where you can begin your ascent to the summit across the various paths and tracks criss-crossing this side of Greeba.
Alternatively, if you begin to follow the fence southwards for a short distance you will then find to your right a large opening in the trees; if you follow this path, you can work your way through the plantation quite pleasantly (so long as you don't get lost) - but it's quite simple. All you need to bear in mind is the general direction you need to be bearing in - that of the gate, as included in the waypoints below at N 54° 12.779 W 004° 34.688. So long as you have this in your GPS and the compass feature in use to point towards it, you should be fine!
Another possible route would be to re-trace your way along the fence back towards the gate (and the properly-surfaced track), and then if you follow this path then you could pick up my Greeba Loop No1 along the way. It's just on the other side of the hedge, and to continue up to the top of Greeba (for this cache), you'll need to return to the other side of the hedge (there's a handy gate nearby), follow the plantation to the bottom corner and then turn left, along to the gate I mentioned earlier.
The cache is a small container, with just a logbook and pencil. It is, as the title suggests, right at the top!