He's suffering separation anxiety. Dogs are pack animals and in the wild will rarely separate from the pack. In the human world it's a different story. In theirs, it's a strong survival instinct to alert the pack to their location if they get lost. It's not crying as such, more a 'Um guys? You forgot about me' noise. 4 Months is going to be tough for him to learn about this strange world and being left alone will feel very unnatural for him. All he's known so far is mummy dog and constant oooh, aahhhh, and attention.

Take time out to spend working on it and leave him alone for a few seconds at first. Leave him inside while you get something from the car, close him in the kitchen while you lay the table. Just little things to get him used to accepting that when you're gone, you'll be back.

After a while start making these sessions longer. Read a book, watch TV in another room with the door closed. But this time wait as long as it takes for him to be quiet. As soon as he is quiet, open the door and allow him to know you're there, bring him through to join you, start playing etc. While he's making noises, don't show any attention good or bad as in his eyes, there's no such thing as bad attention. You need to teach him that absence is only temporary and not to worry. Just ensure he has access to drinking water during the longer stints as you could be there for some time initially.

Dogs don't understand holidays, jobs, nipping out to the car, going shopping. All he sees is that you're gone, and he doesn't know whether or not you're ever coming back.

I threw mine in the deep end as we had lots to work on. We started with nights and kept him outside the bedroom. The first night he whined for ages, the second it was just for an hour. By the 4th he accepted it and stayed quiet. 2 MONTHS later he's brave enough to leave the bedroom door and wander downstairs. He can still smell us so it's not so bad. When leaving the house he waits by the door for 2 minutes, whines for 5 then lies down quietly. (the joys of a digital camera)