That's right. Both legs 4.25" long but half the width.
Slot loading happens when one edge of the port is longer because it uses the enclosure wall as one of it's walls. This effectively extends the port length beyond what you have calculated for. The typical adjustment you make is to account for an extra 1/2 width of the port and assume it's going to be added to the physical length you have in the enclosure.
I attached a picture. In the top box, if the port was 4" wide and 6" long, you could calculate tuning like normal. The port walls are in red and the effective port length is in green. It would act like a 6" long port. In the second picture if the port was 4" wide and 10" long, the green line that represents effective port length goes beyond the physical port length in the box because the air mass "drags" along the back wall. You can see that the two red lines representing the port are different lengths, with the back wall acting like a longer port leg.
If you want more info you can google "port end correction".