The real issue is the turn on current surge of the bulbs. Incandescent bulbs draw a lot of current when they first turn on until the filament warms up, then the current draw drops dramatically.
Dimming the lights will reduce your average current, but the peak current is still going to be there and the triac actually has to work harder when the dimming is at 80% than it does at 100%... If you want to save the triacs, never let the level go below about 5%. By keeping the filament warm, you significantly reduce the turn on current surge.
The Best solution would be to split the strings across more channels. If that is not possible, the 4.2 A will most likely be ok, but know that you are really pushing it. My fear is that everything will be fine this year and next year you will assume 4.2 is fine and want to know if 4.5 is ok. That's just human nature.