MithrasRahl said:
Our DM plays in a very low-magic world. Magic items from the DMG cost 2-2.5 times more than they should, and we rarely find useful magic items, or even gold with which to buy magic items.
While I don't prefer this kind of game, many DMs here on ENWorld run successful campaigns based on this premise. So the campaign is not in and of itself problematic.
We had a recent encounter with 4 Iron Half-Golems, and the entire party almost died. The only reason we survived is because our resident power gamer happened to roll particularly well, and the DM rolled particularly badly.
These things happen. Some encounters push a party to the brink. This in and of itself is not indicative of a problem.
After the combat, he laughed and said "These guys are only CR4! how crazy is that?"
Here is an actual issue. The DM from his comment seems to still be judging encounter difficulty based on standard CRs, when he's modified his game so as to make them relatively useless. Without a standard array of magical powers, he's going to have to eyeball each encounter and guage the difficulty himself.
I'd suggest that you politely and in a friendly fashion point out to him that the CR rating is built on the assumption of standard wealth, and therefore he may need to re-evaluate monsters based on his own judgment, rather than relying on CRs.
I honestly don't think there's a serious problem at this point. Your DM may be quite willing and capable of tailoring his monster selection to your actual power levels. His comment indicates that he may not have thought of this necessity thus far, but does not necesarily indicate he is unwilling or incapable.
Give it some time. If, after five or six more sessions, he's TPK'd you guys several times and still clings to CRs as a justification for his choices ("These guys are only CR6! They're perfectly appropriate!") then you have a legitimate issue to deal with. Until then, consider it a DM's growing pains, as he learns how to handle the kind of campaign he wants to run.