Add mutliple Lists to one list

I have multiple functions that returns a List of objects. How can I add them into one list.

var List1 = GetList1().RunFilter1();
var List2 = GetList2();

The AddRange() function gets far to messy.

List1.AddRange(List2.AddRange(List3.AddRange(List4.AddRange(...); 

Is there a pattern that I can use that it will be easier. I also have extension methods (Filters) that apply to certain lists. Which I interchange based on requirement. Something like this:

var CombinedList = GetAllLists(GetList1().RunFilter1(),
                               GetList2(),
                               GetList3().RunFilter2(),
                               GetList4() ...);

Keep in mind that the GetList() functions being fetched might change.

Thanks for any help!