ode45 should be called outside the ODE function. Your actual ode function is this
function dv=Question3(t,v)
% dv/dt=8-4
dv=4;
end
and then you need to call ode45 from a separate script
to=0;
tf=100;
tspan=[to tf];
v0=0;
[t,v]=ode45(@Question3,tspan,v0);
plot(t,v(:,1));
xlabel('Time(s)');
ylabel('Volume(l)');
