Based on your description, I'm not exactly sure how you're planning on getting the tank water to the drain or where your sump is located but it sounds like it will be gravity fed. Also, being that you have a mixing station in basement it sounds like you may have an understanding about plumbing.
The first thing you're going to want to get are a 4x4x2 or a 4x4x1.5 hub sanitary tee. If you plan on using 1" pipe then you'll have to use a reducer. You're also going to want to put in a p-trap just before the sanitary tee. However, know that going to 1" pipe will eliminate the option of a PVC trap. Depending on location of existing 4" pipe and how you want your new drain line to run into that pipe, you'll be using one of these tees.
As for the p-trap there are a couple of ways to go with that too. If you are using 1" pipe, you can elect to use a standard p-trap. One like you see used with kitchen/bathroom sinks. Or you can stick with 1.5" PVC and use the following p-trap. If it were me, I'd stay with 1.5" pipe the whole way and use a PVC trap.
Should you elect to use 1.5" PVC pipe and the p-trap above then you'll want to install a union before the p-trap to pull it apart in case of emergency. Chances are you'll never have to do this but it's better to have it and not need it than to need it and not have it. Here is a picture of ther union.
I don't know if this helps or confuses you but I hope the latter rather than the former. Feel free to ask should you have any questions.