Stage 4
Hmm. Wet blending isn't the easiest of techniques to demonstrate in stages! Okay, so this is the post-blend stage.
To get from 3 to 4, I've got Neutral Grey and Payne's Grey on the pallet at the same time. For both, there's a blob of straight-from-the-tube paint, and then I dilute on the pallet around that, from just dampening it slightly (near the blob) to making a very dilute wash (further away). So for each colour, I've typically got a range of consistencies going.
I pick an area of Payne's Grey, and go over it a second time with the first brush, applying more of that colour. The second brush has got dilute Neutral Grey on it, and gets dabbed across raised areas and boundary areas. Then it is turn-about with the brushes, mixing the paint on the model while it is still wet, and picking up more and more dilute paint of one colour to try and make the gradation look natural. Brushes with a slightly broken tip are ideal for this.
Stage 5
When everything from Stage 4 is safely dry, I mix Neutral Grey and Winsor & Newton Titanium White, and apply it to the lower legs and forehead. I grade this into the darker grey of the surrounding areas with a thin wash of the Neutral Grey-white mix. If that doesn't look quite how I want it, then I grab the second brush and wet-blend it into more Neutral Grey.
The eye has been done in Games Workshop Chaos Black. The muzzle has been given a wash in black to pick out the nostrils and mouth.
You can, if you want, declare your grey horse finished at this stage, and not bother with any dappling.