That is the exact opposite of what you should be doing.
You should develop code on a system with lots of resource and then cut this down when you know exactly how much resource you need.
I totally agree with that, except that I already need more pins than what the Uno has.
Maybe I could regroup some functions under the same buttons but every button already has more than one function.
This is because you declared the variable array as having three dimensions and yet when you used it you only gave it two indices.
I think I finally understand what you meant ! I thought I needed to declare strings of bytes
and didn't understand the second argument of bitWrite, which was talking about a numeric value.
so I guess I need to declare an array of 4 long unsigned int, that way:
uint16_t array[4][4];
void setup() {
for (int i=0;i<4;i++)
for (int j=0;j<4;j++)
for (int count=0;count<16;count++)
bitWrite(array[i][j], count, 0);
}
Is that right ?
As you have not picked up the hint
ahahah 
I didn't fully pick up the hint indeed. I knew that was a lot but didn't know it was THAT MUCH.
The weird thing is that I thought I had seen much more in other programs than mine,
but they were writing to EEPROM (that I don't know much about) so that must be the reason why 