Varying your workouts

Hey guys, I was just wondering what you guys do to vary your workouts to stop the plateau effect. Is changing the reps from 15 to 5(with a heavier wieght of course!), or speeding/slowing down each rep legitimate variations?
Also, I read somewhere that you have to vary your workout every 4 weeks is this true…if not, how long should I go(approxiamately) before I do a different workout?
As you can tell, i’m a begginer and would really appreciate any imput from you guys.
Thanks

i suggest you stick to the same exercises for the whole time-it makes it more efficient