Just mention it in the body of your correspondence.
Or if you must put in the signature block, your rank doesn't matter anymore, so don't include it. (The first poster is right, you CAN include rank, if you retired after 20 years, but not if you just did four years and got out.) Just put
Joe Snuffy
Army Veteran
Or
Joe Snuffy
OIF Veteran
Or
Joe Snuffy
10th Mountain Division Veteran
Whatever applies in your case.