First, do NOT measure right after the trim but wait 2-3 weeks (mid between the trims) to see how the hoof grows. If you get it right after the trim and your horse grow lots of hoof you'll have problems putting it on after 4-5 weeks after the trimming (friend of mine run into this problem so he ended up do little edge trims to fit it in after 4 weeks)
Second, 4 1/2 and 5 gives you just a size difference, which is OK (per easy care site). In this case you have to go with the bigger size. Also sorry for asking, but are you sure your farrier doesn't leave too much toe on horse?
My horse has difference in 2 sizes (other way around though: her hoofs are too wide). After trying cavallos I got back to Easyboot and got Epic and it fits her nicely (my other horse have oval feet with the perfect easyboot sizing and I put Bares on her).
You can also order them and TRY (just be sure the hoof is very clean so the boot stays clean): if it doesn't fit you can return it (that's what I did with cavallos).