For how long do four 1.5-pound lobsters need to be boiled?

It will depend on how well done you want your lobsters and what you're using them for. If you just want to eat sweet, fresh lobster, bring a large pot of water to a rolling boil (I use a canning pot), add the lobsters head-first as quickly as you can, and let the water return to a boil. When they turn completely red (Maine lobsters) -- usually about 10 minutes -- they're done. Lift them out of the water by the body, allowing excess water to drain back into the pot. Allow to cool for a few minutes, then turn them over and cut through the bottom of the tail carapace. Using a hefty fork, pull the tail meat out from the end of the tail.