Rear Derailleur Replacement on Specialized Allez

[sjta66]

After an over-zealous limit screw adjustment sent the derailleur into the spokes, the derailleur needs to be replaced.

The components on the bike are shimano 105; however, the rear derailleur is Tiagra. Current model is RD-4500.
The bike is a 2006 Specialized Allez
TRIPLE front chainring
Rear Cassette: 12/27
Front Chainring: 30/39/50

Looking to get a compatible model in the shimano 105 set.
After searching around the web I wasn't able to determine what exact model I need. Ideas?

EDIT: would this work?
http://www.amazon.com/Shimano-2010-Road-Cycling-Derailleur/dp/B000XPV4ZM

[Painkiller]

It would work fine, be sure to check hanger alignment before installing. it may have been out of alignment already. It is pretty hard to break one the way you describe. also check your spokes and true of wheel. good luck

[darkbdk]

My gear shifter says gear #1, but its on my 3rd crank. Is there a way to fix this? It is a Enix Mongoose.