If the lameness starts right after the farrier, it's reasonable to assume there's a farrier problem. Also, it's a lot cheaper to have the farrier come out and check it out and do a hoof test than to call out the vet, especially on the weekend. Even if it's an abcess, the farrier is likely going to be involved in the treatment either with a packing, pulling the shoe or putting in a pad or gel under the shoe once it's drained. I always start with the farrier.
If you only have one horse and not a barnful, the farrier may not hop as quick as you'd like but really, one that's been caring for your horse for 4 years should NOT be making you wait 4 days for a reset. In 4 years with my farrier I can honestly say I've not had to call him back ONCE for a reset in all that time. I've been using him for 7 years and I have a barn of 20 plus horses and he's the only farrier. That's a lot of chances to make a mistake.
I have fired a farrier for consistantly soring my horses (the cowboys out here kind figure that if a horse isn't ouchy for 2 or 3 days after trimming they left them too long), punching my horses with a file, and one for drugging an antsy mare without my permission. She was pregnant and she has never been bad for the farrier, just that one, so when he drugged her I flipped and sent him packing. She has not acted up for a farrier since.....hmmmmmmm.