Start with what we know: Our lowest number is [tex]p[/tex] and we want a sum, let's call that [tex]S[/tex].
We know we want consecutive numbers and we want 4 of them. Numbers are consecutive if they come "one after another", that is to say: we can get from one to the other by adding or subtracting 1.
If we start with [tex]p[/tex] we can get consecutive numbers by repeatedly adding 1 (notice we aren't using the "or subtracting" part here because we know [tex]p[/tex] is our lowest number).
So: starting with [tex]p[/tex], adding one over and over until we've got our four numbers we have:
[tex]p[/tex]
[tex]p+1[/tex]
[tex](p+1)+1=p+2[/tex]
[tex](p+2)+1=p+3[/tex]
And now we can write our sum:
[tex]S=p+(p+1)+(p+2)+(p+3)=4p+6[/tex]
