As much steady, calm and gentle exposure to those things that spook him - at 4 he is still a baby and may not have had much exposure to the world so far. do some ground work so he respects your personal space when you are leading. Stay calm, let him look, don't rush him and don't look at the object yourself. use your body language to show your complete boredom with the situation, ignore his reaction as much as you safely can and breath to keep your heart rate regulated. Once he shows signs of calming - lowering head, licking/chewing (not on a physical object) move on as if nothing happened, preferably away from the object as this is his reward for calming down, then re-approach if necessary. Remember each one of your reactions, whether positive or negative reinforce his belief that there is a reason to react. Too many people start flapping and kicking if a horses spooks, pushing them toward the object and just raising the red mist in the horse even further, when they are much better off just waiting. needs patience in the short term but pays off in the long term. Make sure you wear a hard hat and gloves if he is likely to rear and pull away if you are leading him.
There's a big difference between a horse that's anxious or tense and one who's spooky. The tense horse is often wary of contact with the mouth, flanks, or hindquarters, and is over-reactive to leg aids. Teach your horse to come into a more grounded, connected form of mental, physical, and emotional balance. This can be done with simple ground-work exercises, and under-saddle work. His tense muscles impair the blood flow to his brain, and he can't think clearly. His neuro-impulses are inhibited, which makes him less able to feel his limbs. So exercices are very important.
There is a good website, where you can find many ideas and explications : http://www.meredithmanor.edu/features/articles/nancy/horse-relaxation-techniques.asp
I hope this helps, bye.