If its hot, three strands is fine but 4 is better.
I personally recommended hydraulically pounded wood fence posts. You CAN use metal T or you posts but you MUST cover the tops, or you are just asking for puncture wounds.
I would go with the 5-6 foot post for a 4-4 1/2 foot fence.
I also HIGHLY recomend buying the white fluttering fence flags like these from Valley Vet: Fence-Flags Generic (Electric Fencing - Accessories)
They hang from the wire and provide a stronger visual cue. I'm super crafty and tried to make some out of plastic sheeting and paper clips b/c that's really all they are, but eventually I just bought a couple of packs b/c hand cutting and then hole punching each individual one was harder than one would think.
They just reinforce the fencing and b/c young horses have a tendency to get frequent cases of the zoomies, and they're not awesome at stoping and /or judging distance, the flags help them see the boundries from a distance. Posted via Mobile Device