Expanding Storage

So Ive just built my first nas and i am running true nas scale on it.

i am just about to buy 2 8tb WD Reds my worry is eventually i would like a minimum of four drives how easy would it be to buy the two first then add two more later or am i best just waiting and buying all four at once.

I am sure this has been asked a load in the past so please forgive me.