CRK4[]["Step"[rhs_,t_,h_,y_,yp_]]:=Module[{k0,k1,k2,k3},k0=h yp;
k1=h rhs[t+h/2,y+k0/2];
k2=h rhs[t+h/2,y+k1/2];
k3=h rhs[t+h,y+k2];
{h,(k0+2 k1+2 k2+k3)/6}]

fixed=NDSolve[{y'[x]==2×x×y[x]+x,y[0]==0},y[x],{x,0,0.4},Method->CRK4]

Plot[Evaluate[y[x]/.s],{x,0,0.4},PlotRange->All]