As far as I'm aware, the only box still in the wild that runs a 4.x firmware is the NowTV box in the UK, which has a controlled channel store, so I don't know why you'd need to specify 4.x firmware compatibility.
That being said, there's nothing that ties a firmware version to the box you package on, so it's not clear what you mean. Where are you trying to specify the firmware version that it's not letting you? If you're referring to the drop down on the channel submission page, that box will only allow you to choose a firmware equal to or greater than the last firmware version you published against. So, if you previously published a package with a firmware version of 5.1, you'll be limited to 5.1 as the lowest from that point forward, unless someone at Roku makes some adjustments on their end.
My Channels: http://roku.permanence.com - Twitter: @TheEndlessDev
Instant Watch Browser (NetflixIWB), Aquarium Screensaver (AQUARIUM), Clever Clocks Screensaver (CLEVERCLOCKS), iTunes Podcasts (ITPC), My Channels (MYCHANNELS)
