A fine lens for sure.
>If it's sunny ISO400 is good or should I go down to 100? I shoot AV 95% of a time.
>>and f5.6 for birds up to f8 max.
First thing,,, your lens is very sharp wide open,,, no need to stray from F/4. Second thing.... shoot RAW. I always shoot AV unless I'm in a covered stadium etc with 100% predictable/fixed lighting. You need to get yourself 1/1000th sec or faster with the lowest ISO possible.
So,, If the horses are backlit (you're facing into the sun) ,, then use AV,,, F/4,, AI Servo,,, Center Weighted metering +1/3 EV ,,,, ISO 200 should be plenty.
If the sun is at your back (preferable!),,, use AV,,,AI Servo,, F/4,,, Evaluative metering,,, ISO 125.
You can adjust the ISO upwards as conditions darken.
For something 'reasonably predictable' like horses,, I wouldn't bother with back-button AF or anything,, just go with the default method.
Of course,,, this is just what I'd do! Opinions are like nipples,, everyone has at least one or two,,,, I'm sure there are folk here who would do things differently! 