It has four pyrroles; with a five-membered ring containing N atoms joined to give a larger ring. And porphyrin is stabilized by the aromaticity in the entire compound. I suppose the aromatic character of porphyrin enables the the stabilization Fe (IV); where the four lone pairs of the N atoms are involved in the coordinate bonding to the metal.

As we are aware of the mode of bonding between the metal and nitrogen atoms of porphyrins, the cavity in porphyrin ligands can be made just sufficient to fit the Iron in its higher oxidation states like +3 and +4. As the oxidation number of metal increases, its size decreases and fits in the cavity.

This is observed in the Hb and Mb. In high spin state Fe(II) does not fit in the cavity of porphyrins and so Hb has bowl like shape, where as in low spin the same Fe(II) fits in to the cavity Hb has disc like shape.

As we are aware, the metal is bonded to the four nitrogen atoms of porphyrin. Whether the metal sits in the cavity of the porphyrin or not depends upon the size of the metal which in turn depends on the oxidation state and spin state of the metal.

For example high spin Fe(II) does not fit in the cavity of the porphyrin in hemoglobin (Hb) where as low spin Fe(II) fits in the cavity and Oxy Hb has disc like structure. Once it sits in the cavity, the metal becomes stabilized. Thus higher oxidation states of iron like +3 and +4 can be stabilized by creating appropriate cavities in various types of porphyrins.

Pyridyl type ligands are known to be quite good sigma donors, thus they stabilize electron deficient metals such as Fe(IV) especially that they don't possess pi-retro-donation thus they have a good stabilizing effect on the metal centers.

Also thanks Richard for your nice schematic explanation, hope to inform us with more information

Iron (iv) [d4 system] is hard acceptor and porphyrin ring containing 4 hard donor N atom made the complex formation stable one according to SHAB principle. Moreover the chelate effecf favors the Fe-N bonding in Iron –Porphyrin coordination.

As the oxidation states of the transition metals increase, their orbital energies are lowered, giving better energy match to the ligand orbitals and hence stronger covalency and a greater tendency to adopt a low spin configuration (in terms of trends I often teach undergraduates that the crystal field splitting increases with increasing oxidation state). In the case of low spin Fe(IV) we have a t2g^4eg^0 configuration under octahedral symmetry. With a partially occupied t2g set then the Fe(IV) centre can strengthen its bonding through interaction with both pi-donors and pi-acceptors (which mix with the t2g orbitals through p-pi/d-pi bonding. I'd also note that the strength of metal-ligand covalency is substantial in high oxidation states so it is often the case that the 'metal electrons' have considerable ligand character. In the redox reactions of iron-porphyrins it is commonly accepted that some of the redox character comes from addition/removal of electrons which are ligand based, so formal assignments can be deceptive.

because with four donating Nitrogen atoms they can increase electron density on iron and stabilze (IV) oxidation state of it.

I might offer a slightly different take. Specifically, it is important to remember that oxidation states are in the truest sense an assignment we make and not necessarily an accurate statement of charge on an atom in a molecule or complex. With this perspective in mind, it is been suggested in at least some cases that the iron is not in the +4 oxidation state in such systems; rather the porphyrin ring itself is oxidized by one electron [Fe(III)porp(+•)]. Of course from a real MO approach, even this assignment is somewhat arbitrary given the mixing of metal and ligand orbitals such that assigning an electron as coming from one place or the other becomes subjective at best.

Highly nonplanar porphyrins will do that. The classic example for your problem in catalysis which I think has been published would be 2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetraphenylporphyrin. I seem to remember even an Fe(IV)complex of similar systems which are stable in water.

Based on my knowledge, you make a mistake because Iron(IV)-por is not a stable form of porphyrin and no one even could detect it until now, it’s just a suggestion in the catalytic cycle of porphyrin. If “you need to stabilize iron in higher oxidation states in order for it to function as a catalyst for hydrocarbons oxidation.” As you said, I should remind you that making a Iron(IV)-por in lab, is impossible! EVEN oxidation state of Iron in "FeTPPBr8" is (III). [I myself  had been Prepared MnTPPBr8 in lab]