Formulas which help with sequential data

I am trying to find a formula that helps me do this in several rows:
4 rows of the info below:
SO09102015a
SO09102015a
SO09102015a
SO09102015a
Followed by keeping the same letter and number format, but making the last character sequential in alphabetical order, so:
SO09102015b
SO09102015b
SO09102015b
SO09102015b
SO09102015c
SO09102015c
SO09102015c
SO09102015c

and so on...I have over 200 rows of data that I really don't want to have to copy and paste. Dragging with the fill handle doesn't work either.

Hi,
plz see the attachment.