I think this right here shows a serious disconnect in the CR system and encounter guidelines in the DMG, and it's likely none of the designers thought of it. You have two pretty much contradictory parts:
1) encounter strength
So, the idea here is a solo CR X vs CR X is supposed to be a 50-50 toss up. This much makes sense. Supposedly, level 10 PC A fights a fire giant with a 50% chance of victory. Level 10 PC B fights the fire giant with the same chance. PCs A and B fight each other with the same odds and two fire giants fight each other with the same odds. Cool.
Now, the "normal" speed bump encounters are four PCs against one CR monster. This is supposed to be a minor drain. Also, cool.
It stands to reason if 1 CR 10 vs 1 CR 10 is a 50-50 toss up, then 4 CR 10s vs 4 others is also a 50-50 toss up.
2) encounter per day guidelines
They want you to fight encounters that would be a 50-50 toss up fairly often. This is in a world where running from combat can be impossible through no fault of your own.
And that's the disconnect. The system posits both of those are true. That's why I said I'm not sure what you're supposed to do in those encounters. Knowing that the fight is supposed to be a 50-50, I'd want to run. Personally, I'm too risk adverse to throw my PCs up to a coin flip toss.
Of course, one example can be to optimize the crap out of your PCs to be able to handle this, but then you're really not functioning at CR X anymore. You're basically trying to subvert the encounter guidelines by functioning above your level so that you curbstomp all of the normal speed bump encounters and you have a moderate challenge on the deadly ones. It's workable so long as the DM isn't optimizing too.
They really didn't understand the words that they are writing. Fortunately, there are a few things that make a difference here.
1: While even CR 1 on 1s are supposed to be a mirror match, they really aren't because PCs have more resources in the form of items and the like.
2: The same applies for party vs ECL +4 encounter, which could be a mirror party or one big thing or something in between.
3: The 4 on 1 gangups are supposed to use 20% of resources. Not only do they not do that, they can't do that as you physically will not have enough actions to use that many resources. Not to mention, where do you define 20%? If it's HP, is that before or after accounting for healing? If before it is nowhere near 20%. If after, people will literally be dropping dead several times a day as there is no way you can heal them fast enough to compensate for that kind of damage. Mostly because the kind of damage it would take to make the party lose 20% of their HP after accounting for healing would flat out kill at least 1 PC a round every round before they even have an action to heal with.
So when you do encounter your 2-4 Fire Giants at a time, which has a nearly 50% chance of occurring at least once on any given day you have more of an edge over them than the system lets on due to you having more resources and such. At the same time though they still kill 1, possibly 2 of you a round every round. Which means you really need to get in there and take them out incredibly quickly. If you haven't taken out all of them, or at least all but one before they get a turn, someone will die.
There are plenty of ways of doing this. But if you're a Fighter, you're stuck trying to get at that last HP. To do that you have to knock the others out of the way. At this point either you're optimizing enough to one round the Fire Giants, or you're not really helping.
You're not expected to run from encounters unless the ECL is yours +5 or higher. Everything you said about running not being possible is true though. Beating the two to four Fire Giants, even accounting for the strict time limit is not especially difficult.
Contributing to their defeat as a Fighter however is. And Fire Giants are a weak CR 10 enemy. This gets worse if you make the encounter out of better stuff.
skydragonknight: Having someone in the way means a -8 to ranged attacks. 4 for cover, 4 for firing into melee. You can get around those, but I wouldn't really say it helps ranged to have something in the way be it a Fighter, summon, pet...