You are very much a beginner.
Very, very much.
That's not a bad thing whatsoever, but it's a fact.
Using the same weight for all sets is one method, but the way you describe it, that would've just been written as 4x8.
When I talk about or advise X sets in Y-Z rep range, I mean to begin with a weight that lets you get the higher end of reps, and increase the weight as you go as long as you're still within that rep range.
So to use your example, let's say I'm doing 4x5-8 on an exercise and my 5RM is 100 pounds.
Set 1 60 pounds. I get 8 reps, pretty easy. Nice warm-up.
Set 2 75 pounds. I get 8 reps, a little tougher.
Set 3 90 pounds. I get 7 reps after a grind, almost failed mid-rep.
Set 4 I could either stay with 90 pounds or I could increase to 95 or 100. It doesn't matter as long as I get at least 5 reps.
Next session, I'd increase the weight as long as I'm still getting no more than 8 reps and no less than 5 reps per set.