Help needed with a formula to convert upper-case words only to lower-case

[holmwood]

Hi,

Can you think of a formula that will convert upper-case words longer than three letters, into lower-case. Leaving the rest of the string unchanged?

For example,

Mary and John at IBM thought this was a GREAT OPPORTUNITY. Becomes:
Mary and John at IBM thought this was a great opportunity.

I have also tried using morefunc's REGEX.SUBSTITUTE but I couldn't get that to work either!

Any ideas or help would be appreciated, thanks,

Chris