How does nitrogen be in a +4 oxidation state? Nitrogen has 3 electrons in p orbital and and 2 electrons in s orbital and there is also absence of d orbitals so the nitrogen atom can only donate both the s orbital electrons or none of them.

lets take an example of N₂O₄ in this compound oxidation state of nitrogen is +4. oxidation state is the apparent charge on an atom in a compound like in case of N₂O₄ both nitrogen are positively charged and a single nitrogen is double bonded to oxygen atom and single bonded to another negatively charged oxygen atom and single bonded to another nitrogen atom. as there is no electronegativity difference between two nitrogen atom so apparent charge on both nitrogen atom by them is zero. now for oxygen and nitrogen

oxygen is more electronegative than nitrogen so double bonded oxygen will create an apparent charge of +2 and single bonded negativly charged oxygen atom will create an apparent charge of +1 so total apparent charge generated by oxygen atoms is +3 and there is already a +1 

so total apparent charge is +4

simply, 

the concept of covalency takes in consideration about number of unpaired electron, d-orbital  and all. Max. covalency of nitrogen is also 4