Honestly?
I think you are making a bigger deal out of this for him than you need to.
It is a big deal for you, I appreciate that, but he doesn't have any concept of what it means, and you may be making him feel worse about it by projecting your anxiety.
I think you need to concentrate on the positive, how much fun he will have with nanny, how you can't wait to see him, the present you'll have for him. And don't talk about it too much.
Once you are gone, he will be fine and may even treat you with indifference for the first day after you get back (be prepared!)
Try looking at it through his eyes, not yours. He will be cared for, and will probably be spoilt rotten. He will have a great time. Just make sure you do too!
Have a great time, and you are not a terrible mother if you don't miss your child every second when you are away. It is far healthy IMHO not to miss them for some of the time, than to pine for them the entire time. 4 nights is a long time when you have only left him for one before, but its long enough to appreciate the time off you have, to have some fun with DH, and look forward to seeing him again.
Enjoy!