Bulk Rename Utility

This one is kinda the same as ive seen before on here but a bif different that I need to do.

I Have a bunch of files named.

1.png
1a.png
1b.png

To say:

7000.png
7000a.png
7000b.png

I need them to become:

0001.png
0001a.png
0001b.png

To say:

7000.png
7000a.png
7000b.png

Pretty much all numbers from 1-7000 need to be a block of 5 numbers with the the a/b suffix still there as they are all part of that number set.

The first 3000 files as theirs 3 files per number need leading 0's and that's it.

Whats the easiest way to achieve this?

I'm pretty noob with expressions.