I have Elder Sign Omens (iPad), but I'm guessing the game mechanics are mostly the same as Elder Sign.
With 1 investigator (say Sister Mary), I could always beat Azathoth, scoring 4950 in my last game. The single investigator pooled all resources, clues, everything onto herself. I never failed a single Adventure.
With 4 investigators, the resources are spread out thinly among them, making failed Adventures a common thing (at least 1 in 4). Assists did little: I seldom get 2 or more wanted glyphs in a single roll (which would allow me to save 2 or more glyphs at the cost of discarding 1 die). Items and clues could not be transferred among investigators, making it impossible to re-enact a popular heroic response to dire crisis theme where all adventurers pool resources onto the most survivable adventurer for vital tasks.
That said, here are some thoughts on possible solutions (or impossible ones, rather).
Letting us transfer items between investigators would make the game too easy, as easy as a 1-investigator game. But since we should let them transfer items so as to re-enact that heroic response to dire crisis theme, there must be some other way to challenge the investigators. Ok, so letting investigators transfer items back and forth is NOT a solution.
How about making assists count more? Let assisting investigators share items? Yeah, I believe this would even the odds between a 1-investigator and a 4-investigator game.
A 1-investigator game allows instant pooling of resources, but the lack of assists (multiple focus) makes Adventures more challenging.
A 4-investigator game should restricts us to manual pooling of resources (investigators need to be in same Adventure), but assists (multiple focus) make up for the trouble. On top of that, all adventurers in the same Adventure should be able to exercise their individual unique skills/effects.
How's that? Or am I missing some rules in the gameplay? I'm playing it on iPad.
There are some serious balance issues in Elder Sign Omens, I think. I'm willing to help play-test and also help draw up some maths-grounded suggestions (probability). Let me know?