I m working in a project that have more than 4000 parts and I have to animate them all to assemble to an object, and I want to ask if there are a way to make it assemble part by part without have to animate all the objects one by one.
Thanks
Somebody, please?
Hmmm… <blunt>You express yourself poorly. Your post lacks of information.</blunt>
What do you want to assemble? (If it’s a super-sophisticated pervert sex toy, you can call it a piece of machinery.) 
How are scattered the various pieces to assemble? In a cloud floating all around? Fallen on the floor? Tidied up in boxes near by?
What is your current situation? Are the pieces in their final position or away?
What not?
For the moment, all that anybody wanting to help can say is “Sorry, there is no magic button to animate a bunch of pieces the way you imagine it in your head without saying anything. Pick the first one, animate… and go to the next until they are all done.”
And I’m going to bed to heal my bluntness.
It depends on how you want the “assembly” to take place.
If those parts are mechanical parts that need to fit one after other, you do need to animate them all by hand. For example, nuts and bolts need to fit into separate parts in particular way in sequence. This does not mean for 100 bolts you need to create animation 100 times over. You can animate one action and copy its action 100 times over. There are ways to simplify the process.
If you want parts to just fly in and form into one peace, use Rigid body animation in game engine. Let the parts fall / fly apart then run the sequence backwards.
Well you could make each a rigid body and record them falling apart then reverse the video to make it build itself. Would be much quicker, but 4000 objects is going to be pretty intensive to calculate. If they are simple without tons of geometry it is possible. Google for “blender building demo tutorial”