And 4e can do that if it's something that concerns you. Call a fire a "level 3 medium damage expression" hazard, and have it deal 1d10+3 damage for the entirety of your campaign.
Meanwhile, DMs (like me) who aren't concerned with "damage consistent" fire can have it do whatever-the-situation-calls-for. The rules can support both of us, rather than just one of us (i.e. neither of us has to use a houserule!). I find this much preferable to a system where some hazards get explicit values, and others I'm forced to improvise with no guidelines I can look to.