Exterior door replacement

I started replacing my exterior utility room door (builder's original 1975 door) with a new, pre-hung insulated door.
The area is slab on grade, but for some reason the builder notched out the slab for the depth of slightly more than a 2x4 laid on the flat under the door threshold.
In addition, the builder used a rough opening for a 34" door, but installed a 32" door.
My simple 2-3 hour changeout-a-door replacement project just turned into pandora's box.
I can understand a 2'' difference in door thickness, but what's with the notching of the slab? Was this a common practice? It was obviously done by hand and wasn't at all necessary, since the 2 x 4 simply built the threshold back up to where it would have been - slab level. At this point I've hung the door (with screws) but haven't sealed it in. Wondering If I should get a cca pressure treated 2x4 and duplicate the builder's method, or get a couple bags of quickcrete and repair the slab first.
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Only thing I can think of is that if it was an older door with wood threshold he may have done it to have something to nail the treshold to. The newer exterior doors all come with aluminum thresholds and dont require any nailing. I would remove the old 2X4 install the door up as high as you can against the drip edge and then afterwoulds mis a little motar and shove it underneath being careful not to pop it up in middle.I have used this method many times it works great.
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