What is the reason why orbital 4s , becomes full of electrons before orbital 3d?

This is because the 4s orbital is found on a lower energy level than the 3d orbital.

Since electrons are filled from lowest towards the higher orbitals, the 4s orbital being lower in energy level gets filled first.

Unknown008 is correct that a 4s orbital has lower energy than a 3d orbital. We know that because they fill in that order. But what is cause and what is effect? It would be easier to teach and learn about electron configurations if the electrons just went and filled in numerical order: level 1, then the two level 2, then all three level 3 orbitals, etc. But they don't. To explain this, I remind my students that orbitals like to be filled, or empty, or HALF-FILLED. So, after you complete the 3p orbitals (Argon) you could go directly to 3d and put in the first of ten electrons or you could go to 4s and half fill it followed by the next electron to completely fill the 4s orbital. Half-filled wins. Further on, we see that silver would rather have a FILLED d orbital and a HALF-FILLED s orbital -- that is why silver gives us the Ag+ ion. So to say that 4s is lower energy than 3d might involve a little post hoc reasoning. As with your previous question, a Physicist might be able to give a more complete answer.