I know this sounds silly but can he hear your TV in his bedroom. We had similar problems with my son and one evening he was so distressed I got into bed with him, I realised you could hear all the creepy music (Mr Hellion likes creepy films) coming through the floor.
I can only suggest light on low, and playing a CD of someone reading a story - my son still does this he loves, Alan Bennett reading Winnie the Pooh and - it cuts out outside noises and is comforting. We also have a tankful of goldfish by his bed which keep him company (you can buy a kit with a light for about �30). Haven't had so many problems recently.
I'm not sure they fully understand the concept of dreams at age 4 so it just seems 'real' to them.
Whisky in hot milk works for me it's my favourite bedtime drink lol!