I’ve never used 4 jack stands at one time, but would feel safe doing so if they were sturdy stands that were over rated for the weight they were supporting. If you’re raising the car with a floor jack and you are on a smooth hard surface the wheels of the floor jack should move to maintain the the jacking point and also help prevent the first set of stands from moving while you get the set of stands under the car. Another option you might have would be raise one end of the car and do the necessary work on that end then move the stands to the opposite end to finish the rest of the job.
I’ve made stands for my jack stands so I could get the vehicle up far enough so the engine/tranny could be dropped out from the bottom.
Tester
Wow and Whitey and I get deleted for grammer comments.
Its just me but I would not work under a car supported on 4 jack stands period. It may be safe, but I wouldn’t do it. I’ve done exhaust work with ramps, either front or back, depending. I always just work on one wheel at a time using stands and the jack. It’s not a fun thing to have a car come down on you and I have no desire to repeat it. If a car would have to be suspended like that, it would go into the shop.
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I’ve gotten under cars supported by 4 stands but I leave a jack in place on each end also. There’s no such thing as too much insurance. 
Granted, the odds of a car on 4 stands coming down are almost (key word) non-existent but there’s always the what-if factor.
I think about the Pontiac falling off of the lift at the dealer where I worked and almost crushing the mechanic and service manager. The car had been on that lift for a day and a half before the lift sagged abruptly a couple of inches, one of the flip ups folded, and off she goes to land on the roof.
This one happened with me about 8 feet away and it was like watching a Slo-Mo replay.
Use the four jack stands or ramps with a backup such as three or four sections of fairly large diameter tree cut, of course, with a chainsaw. I will be doing that next year when I need to do a partial auto trans fluid change. The car needs to be level to fill the trans until fluid comes out of the fill port. There is no dipstick.
Another possibility is to use inflated tires laying on their sides, stacked two high if you can, to catch the car if it should fall. Concrete blocks are not advisable as a backup; they could be crushed.
And that what makes a mechanic. One who can figure out how to the job, and do it safely.
Four jack stands holding a vehicle up? Ain’t no big thing.
Tester
I’ve used 4 jack stands many times. However, it seems like the older I get and more unexpected accidents I become aware of, the more I use some sort of backup. It’s cheap insurance.
It’s not so much the stands but what they’re sitting on. Concrete, no concern by me whatsoever. Asphalt, how soft and for how long. If inadequate, i may use a pad to distribute the weight to prevent sinking into the asphalt. Anything else, forget it.
I even have dollies for my stands so i can wheel the cars around the garage. I’ve spent countless hours under them on stands. If the wheels are off, i slide them under the frame (or similar sturdy point) to prevent it from going all the way to the floor just in case. Why not? They’re just sitting there anyway might as well use them as additional insurance …
Whenever possible I prefer to support vehicles on old rims. I have welded several several into pairs that give me plenty of working room. It would be very difficult to push, pull or drag a vehicle off the rims when on level ground. When the vehicles wheels were still attached I have often rolled under cars supported by a shop jack for a quick inspection and overlooked my employees doing the same but when the vehicle’s wheels are off it’s a deadly proposition. If 4 stands are used they should be rated at many times the working load. And I never had any faith in those stands that had the pull out pins. The pressed and welded steel just doesn’t look very safe. I am proud that a few stitches on a few occasions took care of the accidents we had. No one ever missed a day from injury.
I work under cars on 4 jackstands all the time. Sometimes without the floor jack in place too. The only way I see it coming off the jackstands would be if someone is moving another vehicle in the vicinity and hits the one on the jackstands.
The point about one side starting to lean as you jack up the other side is valid - I’ve had to go back and reset the first side after the 2nd side was all the way up. When I wanted the whole car up high, I’ve also started with the stands low, and then gone up to high, corner by corner.
I had a colleague who did some work in a third world country. He did his homework and found that at Citroen 2CV was the vehicle that was popular in the country, so that is what he bought. However, a couple of the other people sent in to do some work in the country bought expensive cars. One of these people had a Mercedes Benz. When he took it for a repair, the local people laid the vehicle over on its side to work on it. That was probably safer than jack stands.
I can’t believe nobody factored in “What if there was an earthquake?!!!”
The last time I paid a visit to the Pull A Part yard there were 2 cars there that had gone down. How this happened or whether anyone was hurt or not I have no idea.
The yard removes the wheels and they block all vehicles up on 4 sets of old steel wheels with one wheel sitting vertically on a horizontal one and welded in place.
This is actually a very solid arrangement to use but several cars went over anyway.
Cherokeeboogieman, interesting you mention this. I live in CA, the other week was under the car that was on jack stands (front only), somehow felt more movement than I could have inflicted and then my wife came out to check on me. Sure enough there was a tiny quake.