How can I suggest a reason for the deviation for the pH of 4 M HCl?

As my answer I put,
The pH of 4 M HCl is negative because a molarity of an acid solution greater than 1 produces a negative pH. A negative pH means that the concentration of H+ or H3O+ ions is greater than 1 mol per liter.

With that answer, I don't think I've really giving a suggestion though... what can I add?
Any help would be great. THanks

What you have said is correct. As to whether or not you should say more depends upon the circumstances. You might calculate it by pH = -log(H^+)