What builds muscle faster; doing 30x 4Kg dumbell cruls or 15x 8Kg?

Answer 1

the more weights you lift with fewer reps is the right answer and plenty of reps your muscle actually grows when you are not lifting and through hydration so drink plenty of water lifting triggers the growth thats why we say the more weights lifted is better

Answer 2

The more resistant, the more building going on...

3 to 5 reps builds muslce mass - use almost as much weigh as you can lift

I could type it all out, but you can find tips on weight training here:
http://www.building-muscle101.com/weight-lifting-routines.html

Answer 3

Higher weight with lower reps builds muscle bulk, lower weight with higher reps tones/ builds lean muscle.