a little puller to pull all 4 lines at the same time
But I recommend doing it one at a time
I've used a 6:1 tackle left over from the windsurfing era.
Hanging my weight (~82kg) on it could be too much for 360kg lines, so I just pulled it hard: the average fit man can pull with a force of about 35dan/75#.
This produces a force after friction loss a bit over 450lbs/210kg, plenty to prestretch lines.
A minute or less, not hours, will do the trick.
Once it stretches, it is stretched.
You really only need/want to put 60% or so of rated load into the line.
If you needed to prestretch a heavier line, you could just string it between 2 very strong points, and stand on it. This would give a mechanical advantage of 7:1, if the line was "straight" in the first place. But it also would place you in the firing line of the broken line, if it snapped, with 1100lbs/~500dan of tension in it.
Keep in mind a bowstring with 20-30dan will fire an arrow that will kill a moose.
So whatever you do, do not hook a line to be stretched to anything that could become a projectile.
Attach it to another, stronger line, tied around a tree or something that will not move.
And lead the tail of your tackle off at an angle so you are out of the "line of fire," never hurts to be cautious.