I am going to a party for a 4 months young baby and I would like to hear some ideas what you think would be good presents for him. Nothing expensive though (less than 30 euros or 35 dollars).

Without knowing what the baby already has, it might be a bit of a challenge in not duplicating things.
 
If you're buying clothes, always get it in the bigger size range (in this case....9 - 12 month).
Possibly a musical toy that's good for a 1 yr old might work well.
A gift card at a baby store is also useful as then the parents can decide what's best.
Whatever item you do buy, you can/should attach the gift-receipt..........in case they want to get something else instead. 
 
My little girl just turned 1 a few months back, but a lot of the toys/other stuff she has are from the left overs of her older 3 siblings.  

Thanks for the ideas Mesh. I am simply lost when it comes to buying gifts especially for a little one.

Hehee. Thanks for the tip Paulo:)

That is easy.
A Lego Primo Camel.
It has a music box in it.
Babies like it.
 
 
 

+1 Mesh
 
And call relatives (what our family does) to prevent duplicate gifts. Gift card (as Mesh mentioned) is the other alternative to prevent duplicates.

kakku
I am going to a party for a 4 months young baby and I would like to hear some ideas what you think would be good presents for him. Nothing expensive though (less than 30 euros or 35 dollars).

a 4 month party??!! boy those parents are setting themselves up for an expensive childhood...

I have 4 kids. The toy that they liked the best at 4 months were the toys that dangle and slowly spin above their crib. Something they can also reach for.Sounds when touched got a reaction. Laughing cooing etc
 

bayoubill
I have 4 kids. The toy that they liked the best at 4 months were the toys that dangle and slowly spin above their crib. Something they can also reach for.Sounds when touched got a reaction. Laughing cooing etc
 

But wasn't that a bit hard on the animals you hung up there?