I had this problem, with "replacement" being the only solution offered by Anova ($200 for out of warranty). I had a time of it, finding the fuse "drawer," and the drawing makes it a little odd. #3 and #4 are actually in the same, black receptacle.
There's a black 120V AC electrical connection which accepts the female end of the main power cord. It has 3 male prongs, and looks like a standard socket you'd see at the back of a computer.
If you look Very Closely at that receptacle, you'll see the outline of the fuse "drawer." It's located at the #4 position in the drawing, but it's actually a part of the AC receptacle. Then; if you look just inside the recessed area of the receptacle, where the three male prongs stick out, you'll see a small indentation just on top of this drawer. That's where you can fit a small screwdriver blade or butter knife blade and gently push outwards. That will easily pop out the little drawer.
In the fuse drawer, you'll see the main, working fuse held in two clips. Next to it, you'll see another fuse (the spare) with no contact clips. It's just sitting there, easy to take out and use to replace the one in the clips.
The fuse is a 20x5 (20mm by 5mm) Fast-Acting 10A (ten amp), 120V (one-hundred twenty volt) fuse. I had to do a lot of research, then have a professional electrician examine the blown fuse under a high-power magnifying glass. Despite Anova telling people it's a 15A fuse, it's not! It's 10 Amps.
They can be purchased in many places, and I happen to have an electronics parts store in town. The store sells things like cables for TVs, DVRs, and so forth. It's like a Radio Shack used to be. If you have a place like this, you can typically get a 5-pack of the fuses for around $3.50 USD. If not, you can find them online at all sorts of electronics parts suppliers.
I've used my circulator often for over 3 years. The fuse blew, and left a very clean break. The guy I spoke with explained that over time, sending constant electricity through the fuse will degrade the metal. It gets a little old, and wears out. The metal of the fuse just breaks. It's a clean break, with no discoloration. Like mine did. The unit ran for about 5 minutes, then simply stopped.
On the other hand, a short circuit in the system likely will cause the fuse to blow and leave blackened area inside the glass. People have spoken of a blackened area in the clamp area where the AC power cord connects. That's likely due to a 15A fuse being in there, failing to prevent a spike or other problem and burning the equipment. It might explain why power cords are melting in some postings.
Replacing this fuse is so easy, it's clearly designed as a user-replaceable item. There are no instructions with the circulator on the Flash drive or reference PDF file, and I've found no information online. One community member helped a lot!...by mentioning in the FAQ, in passing, the extra fuse. I didn't even know there is a "Community!" This post is to validate that information and provide folks with the specific fuse for this unit. Hope it helps. Remember, you do NOT have to unscrew anything in the clamp, or even use a screwdriver or other tool at all. You only need a way to open the little drawer, and you could probably do that with a fingernail.