Have my Save as or Save For Web Automatically increment

I'm trying to get photoshop to do  the following for me:

When I click "save to web" I want the item I save  to automatically be formatted in the follwing method

PhotoItem  PhotoIncrment

for example

0001a.jpg

0002a.jpg

and so on

I'm not sure  of the best way to accomplish this quickly and easily

also I need  an easy way to incrment the number as the photos will be edited like  this:

0001a.jpg

0001b.jpg

0001c.jpg

0001d.jpg

0002a.jpg

0002b.jpg

0003a.jpg

0003b.jpg

0003c.jpg

0003d.jpg....

I will probbably need a button to designate when I am editing the next item

any help  would be greatly apreciated!