A 4 or 6 layer option would be excellent. The design rules have been improving at the batch PCB order houses, I remember when 10mil was a nice feature. I think a good 4+ layer option is only a matter of time. 100% e-test is great as well.

OSH Park has a 5mil trace/spacing 4 layer option. No e-test (Got burned by that one. Always double-check your boards before assembly!) and the schedule is a bit unpredictable.

There are a few options for 6+ layers that don't seem too unreasonable. For example, Pheonix PCB does 100sqin of 6-layer board for a single design for $550. They go down to 4mil trace/spacing and 8mil drill (about $90 extra). There might be better options. At this point, for the hobbyist market, design tools start to be the barrier, not manufacturing. $500 for a bunch of prototype boards seems like peanuts compared to the price of Alitum.