Is there enough time to pour 40 sqf 4" slab with 60lb bags of premixed concrete if working alone? Any hints?
The answer depends on your level of physical ability and ambition, and is therefore mostly subjective. There are plenty of folks who could do it. I wouldn't try it myself at my age and level of fitness. Concrete is really freaking heavy. I would want some additional labor on hand. 

I attempted to finish a sidewalk and garage apron on my own when my help had to leave, and there's little as nerve-wracking as finding oneself racing the clock with setting concrete. You never end up with a good finish in those cases, and if you try something foolish like watering the mix you'll end up with a weak surface prone to flaking and disintegration. 

My $.02: Play it safe and have someone around to help you mix and move the mud.