F.4 Chemistry mole ...

you have a wrong concept.

The coefficients in the equation means the actual amount of reactant incolved. From your equation, 4 moles of NH3 would require 5 moles of O2 for complete reaction. Therefore more O2 is needed. As you said, mole ratio = 4:5, so consider a simple ration problem:

NH3 : O2

= 4 : 5

= 4/4 : 5/4

= 1 : 1.25

= 0.1 : 0.125

so 0.1 mole of NH3 needs 0.125 mole of O2 for complete reaction

Since there is only 0.1 mole of O2, O2 is the limiting reactant.