Started 4 plants from seed -- growing well, coming into bud, 12 in high, in 4" pots BUT bottom leaves are turning yellow and dropping --- how come?
With plants generally this indicates a moisture problem. Review your watering and check condition of potting medium. As potting mixtures age and decompose they hold less air and more water. Abutilon are also subject to sucking pests, particularly whiteflies. But these would tend to be scattered about the foliage, not concentrated on the bottom.
Also sounds like they might be rootbound. That will cause yellow leaves. Check to see if they need repotting. Newt