I don't think the legs are too thin if it's mean to be a quarter horse - looks very much like ideal quarter horse conformation to me ;) - but knee is too high and the eye looks a bit small. It does need more bone for most other breeds. And please do give it teeth and tongue.

The neck looks especially nice.

Will it take P4 horse poses or am I going to have to start from scratch again?

"Do not meddle in the affairs of cats, for they are subtle and will piss on your computer." -- Bruce Graham