Traditionally, it has always been the U2-policy to split all earnings evenly (for example, you see all compositions listing songwriters as Clayton, Hewson, Evans, Mullen, etc.).
I guess around 1987 or so -- certainly with Rattle and Hum -- Bono and Edge started spending a lot more time together, writing, and fine-tuning more carefully crafted songs. When conflict arose in late 1990 in Berlin, Bono and Edge pointed out to Larry and Adam that they had been writing most of the songs and putting in the extra time, and therefore should in all fairness be compensated as such. Whether or not this ever happened I don't know.
If it did, certainly Bono and Edge will have earned more than the rhythm section. But it's still highly possible that all 4 guys have earned more-or-less exactly the same amount.