I can see the thin growth at the tips in your first picture. Your cactus wasn't receiving enough light for a while. I'm not sure what 4 days in a dark room did to it. However, moving it from insufficient light in your room, to a dark room for 4 days, then to outside with full sun will probably result in sunburning the cactus. You need to gradually increase the amount of light or else a plant will sunburn.

I haven't had experience growing this particular cactus but I've seen other plants droop considerably after being repotted. Some cacti need more water than others and are more sensitive to having their roots disturbed. I'm not sure what was done to the cactus when it was repotted but I suspect its roots were disturbed a lot. Unfortunately it looks like a flat pad type of cactus that wasn't getting enough sunlight before repotting so the stems aren't very strong. Add that to disturbed roots and.. well.. you get what is shown in your second picture.

What to do now? I suggest taking it out of full sun. Gradually increase the amount of sunlight if you haven't done that already. Do not give it a lot of extra water. That will likely cause more problems like root rot. Continue watering according to the general advice of letting the cactus dry out between waterings. Though, someone with more experience with this cactus might be able to give better watering advice. I expect eventually the cactus will begin to recover. It might not go back to looking exactly how it looked in your first pic but with good sunlight and good watering I think it will produce new growth. Given time the old growth that doesn't look right like the thin growth at the tips can be clipped off and new growth will take its place.

Why was it repotted? The old pot looks like an OK size for the plant.

Polypompholyx is probably right - plant may be 'in shock'. It should recover. BUT as already said, it looks like you had it in low light before repotting. It should not have skinny pads like that. Did you get it recently? Where is it kept (lightwise)?

Pls. do not take this as more bad news, just as suggestions: I also agree that original pot was big enough. Can't tel from the photo difference in size, but looking at how plant fills up new pot, it seems to be same or very close in top diameter (opening). Recurved type of pots may look nice, but are not very practical: it is, usually, quite difficult to get the plant out of pot that has bigger body than top opening. So when time comes that it really needs repotting (probably quite a few years), and roots are healthy - it could be quite a struggle to get it out. Sometimes, it is best to break the pot. While this sounds like a disaster, best may be to concentrate on getting plant to recover now. Do as poly already suggested - start with plant in original amount of light and move to little more every few days or a week.

Make sure not to overwater: sorry for repeating what was already said, but I think it is very important. Make sure soil deep in the pot is dry before giving it more. Use a bamboo or wooden skewer or chopstick (or even pencil), inserting it deep in the soil (at least 3/4 deep or even more). Leave it in for few minutes, then pull out. If it is completely dry and clean = time to water. If damp/cold and may have soil particles sticking to it, soil is too wet and wait few more days (repeat test before watering).

Do you know what kind of potting mix they used? If not, pls. go back and ask. They should tell you.

Pls. ask should you have any further questions.