Adding notes

I want to add a note at the 4th fret where my cursor is as I accidentally missed it out. I have written lots of tablature after this point so do not want to delete this tablature, just to insert one note and then have to reinsert the tablature all over again. I want this note to be a quarter note, and hence want the tablature after it to be shifted by a quarter note when it is inserted. I hope this makes sense. This is causing me lots of problems. Look forward to hearing suggestions.