Open Middle Sum Problem

Creation of this Open Middle resource was inspired by this tweet from Kurt Salisbury. Using the digits 1-9 at most 1 x each, fill in the top four (or bottom four) boxes to form four two-digit numbers that sum to 100. Each digit repeats once. You will see where it repeats after you start typing digits in the boxes. You can create your own magic sum to try to reach by typing in the input box lower right.

Note: If you accidentally repeat a digit, one (or more) of them will disappear. If this happens, enter another digit inside one of the input boxes that became empty.

What is the highest possible sum you can get here? How do you know?

What is the lowest possible sum you can get here? How do you know?