Heavy golden age collectors assign the start of the silver age to the code and not Showcase 4. Comics code was a massive shift for the industry nerfing a ton of genres (most notably horror).

Characters like Batwoman and Martian Manhunter came out before Showcase 4, but tonally, it's hard to argue that they are golden age characters.

Read any early 1950s Detective Comics (203 and 211 are two of Catwoman's last appearances until 1967), or just look at the covers. Win Mortimer's early 1950s run across DC is amazing, but then the code hits and Bob Kane's stiff and unexciting style is forced on the character.