My son and daughter in-law's home has a room for the baby which gets no natural light. The baby (4 months) now won't sleep unless it is pitch black. This is causing problems when they visit other family. Can he be trained to sleep anywhere else?? HELP PLEASE!

I love how you worded it, "can he be trained". Yes, children can be taught to function in just about any environment. He only sleeps in pitch dark because that is what he is used to. If he is not used to something, it is difficult for him to adjust. Older children can adapt to new situations because they have experienced more. My kids can travel for hours in the car because we have exposed them to long car rides. It was not easy at first, but they are fine now.
One thought is to put a very low watt night light in his room. This will create the difference of light that is found in other family houses. As with anything that a child needs to learn, this will take time. This will also be difficult, because he will probably fuss until he is used to it. He will come along. Think about it, we all slept in pitch dark for 9 months in the womb. We all got used to it. He will be fine.