This?

Fa = 3*V; Fb = 4*V; Fc = 5*V; Fd = 6*V;
a = 1; b = 1; c = 1; d = 1;
matrix = Table[
   If[Fb - Fa > 5, b = b + a; a = 0];
   If[Fc - Fb > 7, c = c + b; b = 0];
   If[Fd - Fc > 8, d = c + d; c = 0];
   {a, b, c, d}, {V, 20}];
ListPlot[Transpose[matrix], Joined->True, PlotLegends->{"a", "b", "c", "d"}]

There is a lot going on in that for someone who is new to Mathematica. {} and ; mean very different things in Mathematica than they do in other more "normal" programming languages. Search for some of those words and operators in the help system, see if you can try to understand why I did some of the things the way that I did, click on "Options and Details" in the help page to see more information.