Interesting how many people still have the trusty old SB4200, I got mine back in early 2001 or so, used it for around 5 years or so before I replaced it, and then gave it to my mom to use so comcast would not charge her for a modem rental. I just replaced the 4200 to a 6120 due to the 4200 dying from a lightning strike, but it lasted 10 years of service...impressive by tech standards.
The 4xxx series Surfboards, IMO, were just built better than the models that came after up until the 6-series. Sure, they were only DOCSIS 1.1, but they were consistent and didn't die easy.
Now though, it's worth going to DOCSIS 3 if your area supports it. Even at the lowest speed tiers, I've found the multi-channel setup to be faster and more consistent under load.
The one big thing I miss about the SB4200 was its internal power supply, so it didn't use a wall-wart AC adapter. I wish the SB6120 used at least a small power brick instead, easier to plug in on a UPS or surge strip.