I have no scientific research to back this up, but I'd hazard a guess that the great majority of bands - rock, pop, and all genres that hover near those genres - have four members. Certainly most of my favourite bands were/are quartets, and most bands I've belonged to were four-pieces. There are exceptions to all rules, of course: from the crowded-stage operations like Lynyrd Skynyrd, Tragically Hip and Allman Brothers Band (6 or more members each) to ultimate lean, mean trios such as Rush, the Police, the Jimi Hendrix Experience, Cream and Husker Du - there are no hard-and-fast rules as to what the ideal number is.
As you've probably noticed, Mike Luno Band is a trio, and has been for as long as we've had the name. For better or worse, I'm not usually one to back down from a musical challenge, and I love the idea of playing in a three-piece band the way a tightrope walker might enjoy performing without a net. It's more work, to be sure - there's no hiding mistakes in the live mix. Arrangements have to be effective with very few ingredients, and in handling all vocal and guitar duties in an evening, it's a mentally demanding gig...but as in many other situations, the greater risk makes for a more gratifying pay-off when it works.
Ah, there's a phrase - "when it works". I take some solace in the fact that musicians far greater than I often wrestle with the phenomena of good performances varying with bad. I distinctly remember a journalist's account of a Stevie Ray Vaughn soundcheck - where Stevie is fighting fruitlessly with his guitar sound until he finally dons his favourite hat, and returns to his stellar form only then. I can't say I've experimented with different headwear (I look stupid in almost all hats), but especially during periods when I'm not playing live so frequently, my performance quality does seem to vary pretty widely. 
There are a few essential ingredients to a good performance: I'm relaxed enough to be thinking in terms of expression far more than I worry about mistakes; my body (especially my voice) feels strong and responsive; and I have total confidence in the people with whom I share the stage. It's no accident that I've played with Curtis Leippi for nearly sixteen years now.
Admittedly, I am sometimes tempted to return to a quartet format - the backing-vocal possibilities alone are quite alluring, let alone how it would lighten my onstage load considerably. I do have a habit of arranging my music with lots of backing vocals (my nod to masochism, perhaps), and the wider sound-pallette might broaden the band's appeal as well. It's comforting to think that the lesser burdens of shared guitar and vocal duties would help me attain the ideal performance mental 'zone' more consistently - but I seriously wonder if it would. I certainly didn't have consistently stellar performances in earlier incarnations of this band when we were a quartet and a quintet at various times. Though I admittedly felt quite overtaxed at a recent performance (adding doorman, soundman, hucksterer, manager and pr director to the night's duties didn't help my performance or my headcold, I daresay), I still see 'the zone' as an attainable goal for all performances, even the singularly demanding ones. Call me the eternal optimist....