k1=1;k2=2;k3=3;k4=4;
l=6;
modell:={x1'[t]==-k1 x1[t],
         x2'[t]==k1 x1[t]-(k2+k3)x2[t]+k4 x4[t],
         x3'[t]==k3 x2[t],
         x4'[t]==k2 x2[t] - k4 x4[t],
         x1[0]==1,
         x2[0]==1,
         x3[0]==1,
         x4[0]==1};
loesung=NDSolve[modell,{x1,x2,x3,x4},{t,0,l}];
    abb=Plot[Evaluate[{x1[t],x2[t],x3[t],x4[t]}/.loesung],{t,0,l}
                   ,PlotStyle->{{},
                   {Dashing[{0.01}]},
                   {Dashing[{0.02}]},
                   {Dashing[{0.03}]}}
                   ,AxesLabel->{"t","x1,x2,x3,x4"}
                   ,ImageSize->{400,300}];