just bought a new 06 exiss xt140 and a new 08 f-450 truck to pull the trailer. its are 1st lq slant trailer. leaving for ohio next mouth for a show and taking one horse a 11 mouth old filly. is she better all the way in the back or more towards the front of the trailer. also looking to load 4 wheeler back there

If I have one horse it always goes in the first stall. If I am bringing the quad, I load it first, then the horse. My logic (?) is that 1: I won't scare the loaded horse by loading and unloading the quad and 2: I can get to the horse in a emergency faster.

I haul one horse most of the time in my 3H slant. I put my hay, straw and stuff in the first stall (some times the second one too) and the horse last.This way i can load my gear the night before, then the horse. No sense loading the horse first only to have them cool their jets waiting for me to get stuff in and then wait until I unload.

I have also hauled my husbands motorcyle along with two horses- put the bike in first- then load horses. Again it just makes sense to get the bike in without bothering the horses.

I mostly haul one horse in a 3 horse trailer and I put the single horse in the 2nd (middle stall) for two reasons:  More room for him to spread his feet out under the dividers into the other stall if he wants, and the weight is more directly over the trailer axles.  In a 4 horse I'd probably use the 2nd or 3rd stall (the two middle stalls) for a single horse.  In a LQ trailer I doubt positioning between those two stalls is an issue, the trailer is already so long the difference in pin weight vs axle weight between the two stalls would be minimal.  Just my thought. 

<edit> Sorry, completely ignored the fact you mentioned hauling a 4 wheeler too.  I agree with the other posters, load the 4 wheeler first and put the horse in a rear stall (3rd or 4th stall).