I am growing my cilantro in a pot, and it gets at least 4 hours of sunlight. I water my cilantro once a day. I also have five other cilantro plants in my pot, which is about 12 inches in diameter, and they are also wilting. I did repot my cilantro as I initially grew it from small pot, but has been over three weeks now. Is there a reason for them wilting and should I be worried?
This is a typical case of over watering. Once a day is too much unless you have high temperatures.
It also looks like you do not have enough drainage in your compost. This leads to poor “particle air porosity”, basically your compost is solid with no air gaps for water to drain. I also think you need more sun. Coriander or Cilantro is a Mediterranean herb and needs more sun. When growing plants, always look at their habitats and growing habits.
Good that you used potting soil, and the pot has drainage holes. However, we can't see the pot, so don't know how big it is, which might be a factor. If it's too large, trying to keep the roots of your little plants supplied with water means you have to keep the soil in the pot too wet, but without seeing the pot, it's hard to say.
What is a factor is moving it outdoors into full sunlight and bringing it back in again - seedlings, unless they grow outside right from germination, should not be exposed to full sunlight, it should be introduced gradually, and nor should they be moved from indoors to outside and back again regularly. If you're moving them outdoors to acclimatize them ready for planting outdoors, just stand them outside in a shady spot for the first few days.
The soil does look too wet, so you probably need to reduce watering - water when the surface of the soil feels just about dry to the touch, but not so dry its shrunken from the sides of the pot. If the pot is standing in a tray our outer pot indoors, empty that 30 minutes after watering, so it's not left sitting in water.