given f(x)=x^4 (iv) stuck

i) complete this identity f(x+h) = x^4+4x^3h+...

ii)simplif (f(x+h)-f(x))/h

iii)find lim h tends 0 of ii)

iv)state what this limit represents

the answer is gradient of (tangent to) curve but i'm not sure why. If someone could explain to me the reasoning then i could apply it to other q's,thnx