I stripped one of four bolts on the faceplate of my stem. Is the stem usable until the replacement arrives?

Dont use the stem , its rated at 4bolts@ 6nm , by putting in the 3bolts you may need to over tighten one of the other bolts to compensate for the same level of clamping force which may distort the clamp and or strip the threads in the other bolts.

Food for thought ,why not consider to repair the thread. Just grab the next size tap/thread and put in a bigger bolt with a matching thread.

PSA : store your torque wrench at zero (nm), if its that spring click mechanism type.It will hold its calibration longer , also helps if you don't drop it too :).

Answer this question for yourself:

WHEN the stem fails, what's going to happen to me - AND EVERYONE AROUND ME?

And only ride in ways that you're willing to live with the answer to the above question. Because when that stem fails, you're probably going to lose control of your bike and have a really hard time stopping - or worse, a really easy time stopping, err, splatting.

Noodling around on the local flat, empty-because-it's-winter MUP at 10 kph? Probably fine.

Hard, fast training on pavé for spring classics with your teammates? Not so fine.

I have no personal experience or engineering background. Based on lay intuition, I would agree with the posters suggesting that leaving one bolt out is a bad idea.

In the interim, there’s a potential solution if you can get to a hardware store. Find a high quality steel metric bolt.

1. You will need to match the thread pitch. I suspect it’s an M5 thread. Find an M5 bolt and hold one of the other stem bolts up to the candidate bolt. If the thread is finer or coarser, it should be visually apparent.
2. You’ll need to match the length of the threaded section. If you have a caliper at home, go and measure the length of the threaded section from the head to the end of the bolt. Else, ask for help at the store.
3. You’ll need to match the type of head. I suspect you are looking for a socket head bolt.
4. This is the tricky bit: you’ll need a bolt that’s strong enough. This site suggests that there are at least three common strength classes for (non stainless) steel: 8.8, 10.9, and 12.9 in increasing order of strength. The site said to avoid 8.8. It gave a recommendation for a grade for stainless steel. If you are determined to get back on the road, then I’d say go for the non-stainless.

There is some risk involved here, and the hardware store personnel might not know the strength grade. Also, steel quality might vary even within a given rated strength grade. Whatever risk there is should be less than riding a stem with 3 bolts.

I say no, the stem is not usable. I recommend considering the real potential for severe harm when the stem fails. The danger will likely not only involve you; I certainly wouldn't want to be riding, driving, or walking near you until the stem is replaced.

Attach a lightbulb to the stem and turn it into an artistic cycling-themed lamp. Don't sell the stem or give it away to the "next sucker".

Here's my personal experience, and a story of caution:

When I was young, I rode a bike with a minor stem tightening issue that I couldn't afford to fix. I thought it was okay until I landed a jump. The handlebars rotated and my chest plunged straight into the stem at full force. Back then, we didn't wear protection. I still remember the impact and the pain to this day. I consider it a miracle that I survived and am still alive.

It depends on your riding style and how the bike will be ridden. With 3 bolts you can easily ride the bike to get you from point A to B, just don't ride the bike in a way that will put pressure on your handlebars. For example, you shouldn't do any tricks on the bike. If you can't avoid putting on the handlebars, then I wouldn't ride the bike.

I often ride my bike using only one hand because I'm carrying something in my other hand. This is one way to make sure you don't put a lot of pressure on your handlebars.