Dieter H. Marschall's "Walther Pistols Models 1 Through P99" (from UCross Books in New Mexico) will provide you with all sorts of history and detail on the Modell 4, including:
Production era: 1919-1928
Caliber: .32ACP (8 rounds)
Total length: 6 inches
Barrel length: 3.3 inches
Weight: 19.4 ounces
Variations: At least five
Number made: Nearly 275,0000
About 75,000 Modell 4s were made for military use. After WWII, this model was still used by the Bavarian police (these were stamped ByP). Like the Modell 3 and the more-famous P5, note that the ejection port is located on the left-hand side.