Is it true that if bees were to disappear from the earth completely that humans would have only four years to live?

There is a quotation: "If honey bees died out, mankind would only have four years to live." This is supposedly attributed to Albert Einstein, though it is very unlikely to be his.
However, this doesn't detract from the seriousness of the situation. Roughly four out of every five plants are insect pollinated, and of those nearly 90 per cent are pollinated by bees. If they were lost very many flowers, trees and vegetables would not be pollinated. Without pollination they would not produce seeds or fruit. Roughly one mouthful out of every three of the food you eat is directly attributable to the work of bees. Without bees, the world would be a lot less colourful, and we would have quite a bit less to eat. There would probably be widespread starvation, but I doubt there would be enough to completely kill off the human race.