A few more questions

At 4 weeks, your temp should be around 75 degrees F in the brooder. I don't know where you're located, or how cold it gets at night, if it's much colder than 75 I would still use the lamp. During the day I would turn it off if it's in the 80's. Chickens do roost (when they're a little older) and sleep at night. They rarely budge til morning once they've gone to roost. You can let them out at whatever time fits your schedule, they'll be happy to go out at sunrise, but it isn't necessary. I let mine out at 9am since I now have two crowing cockerels and don't want my neighbors to turn on me. ; ) As for grit, yes, if they are eating things other than their chick starter you should supply some. If you can't find a chick grit, you can use coarse sand, or some people buy regular poutry grit and pound it to break it up a bit. You should avoid parakeet/canary type grits as they often have oyster shell mixed in and all the extra calcium is not desirable at an early age. Good luck with your babies!

 

I have my brooder in the coop, not the house. I turned the heat off at 3 weeks during the day this year and off totally at about 3-1/2 weeks. Usually I wait until 4 to 4-1/2 weeks, but it was only getting down to the lower to mid 70's at night and in the upper 80's and lower 90's during the day. I had 14 so they could huddle together to keep each other warm if they were uncomfortable, but they did not. I don't know how many you have or what breeds, but if it is in the 80's during the day, they will be fine without the heat during the day. I don't know how cool it gets inside your house at night, but they would probably be OK at night too even if they are not totally feathered out, as long as it doesn't get real cold. Your call though. Your conditions, experience, and risk tolerance are different from mine.

If you put them on grass, they will find their own grit. I think it would be a good idea to give them a little before you take them out, but I start giving mine grit about day 2 or 3 in the brooder, just to get their systems set up the way I think it should be, so I am prejudiced about that. If the sand in that sandbox is the really fine smooth play sand, I would not use it, though it is better than nothing. It is just too fine and smooth to do much good. They really need a coarse irregular sand or small pebbles for grit. Construction sand usually works great. I sometimes collect sand and small pebbles from my gravel driveway for grit, though my usual source is the dirt from the adult chickens' run. Or you might cut a piece of sod and put the whole thing, dirt, grass and all, in their brooder.

Chickens cannot see in the dark. Their eyes just don't work that way. They are active when they can see and not active when they cannot. Semi-dark does seem to soothe them and help keep them calm. On a clear night with a full moon, if they have windows placed right, they can leave the roosts and be active at night.

When do you need to open the coop in the mornings? I don't think there is a clear answer to that question. There are a lot of variables. If you have lots of room with feed and water in the coop, you do not have to be in a big rush to let them out. If they are crowded (I know, lots of room and crowded are not real clear terms) or you don't have food and water available to them when they wake up, they need to be let out earlier. They are going to wake up hungry and thirsty. Their crop is empty so they will act like velociraptors when food becomes available. They can go a while without food and water after they wake up and it won't hurt them, but exactly how long, I don't know. Mine seem to be hungry rather than thirsty when they first wake up, but they do hit the water pretty hard if they are kept from it for a couple of hours.

 

I have 3 2 week olds, in my house right now, the 13 week olds are in the coop, and it is too chilly at night, 40*'s, so I am keeping them inside till they feather out, then place the cage in the coop with the big ones...I started using a light, heat bulb, but after watching them, I turned it off during the day, it was getting into the high 80*'s, but now, even at 2 weeks, no light, night or day...they are doing great and thriving...so it depends on the chicks I think, I know we are supposed to keep them warm, but mine seem to be doing great...