I think there is a misunderstanding: ode45 uses the result from the 5th order RK method for the solution, whereas rk4 is a 4th order method. The "4" in the ode45 means, that the 4th order is used for the variable step selection. Now: for "nice" functions like 2*t, both rk4 and rk5 reproduce the result. Therefore you see no difference. For t^5 you see, that the 5th order is still exact (error of ode45 small), whereas 4th order rk4 cannot give exact result (as you see).