Ok, here how i solved the thing...
I created an instance variable called "block" to store wich one is 1/2/3/4 th block.
And used a local 0 value variable that i reset each time a piece is created.
After creating the piece, i used a "on create" event that add 1 to the local variable.
And set the "block" instance variable to this local var.
And now i have the number of each block created, and can use them separatly !
"on created" events runs each time a sprite of the same type/object is created, this is what i needed to store their ID /number
thanks a lot to this guide here : https://www.scirra.com/tutorials/361/un ... moke-trail
Hope this could help other newscomers, this issue seems to have been asked many times.
But haven't found the answer i needed till now.