Regarding Random Generator

HI all

I Need to Generate String Like tHIs

When 1st User Enter their details like

FirstName

LastName

SurName

CARDNO

Automatically generate the string in CARDNO textbox

The string-based on Surname

My Surname is: MAHESH

the String must be like this AAAAM0001K

First 4 letters in this Sequence: AAAA

AAAB.............ZZZZ and repeat again

5th Letter based on Surname

next 4 numbers in this sequence: 0000,0001,0002.......9999 and Repeat again

10th Letter must be random a-z

String Must be 10 characters

Can any one help this

Thanks