Batch add field, calculate field as filename

So here is my last and final question and unfortunately my largest hurdle. I have a tonne of shapefiles (4249 to be exact). I have never used a model builder or scripted anything. I need to somehow batch add a field (named RTENO) and assign the shapefile name to that field. The only other option I have is to batch calculate field with file name one at a time over 4000 times. If anyone knows of anything I could do to make this possible it would be a life saver.