Question for anyone who knows more about tubes than me -- which is probably just about everyone!  

I'm replacing the rectifier tubes in my CSP3 and ZP3 with OS RCA 5U4G and 5Y3GT tubes.  I've been using an OS RCA 5Y3GT in my ZP3 and the Electron Tube brand Chinese 5U4G, that Steve originally supplied, in my CSP3.  The manuals say that the preferred tubes are ZP3/5U4G and CSP3/5Y3GT so I'm trying those combinations.

Now the question.  My OS RCA 5U4G and 5Y3GT tubes both have 5 pins while the Electron Tube 5U4G (and Valve Art 274B, which shipped with the ZP3 originally) has 4 pins.  Without getting too technical, what's the difference?  My understanding is that the 5U4G tube is a 5 pin tube.  So what is the Electron Tube 5U4G really?

But don't the pins connect electrically to something?