Yep - the mIU/ml or IU/l (same thing) is the concentration that the test can pick up. So your blood test told you you had a blood concentration of 4 a few weeks ago... It would logically have HAD to go UP to show on a 25 IU/l sensitivity test. And it shouldn't go up unless you're UTD - so tentative congrats are in order, but I'd still repeat the test in a few days, and if the line's darker make a GP appointment