I am making a new bar and I am going to play around with making a 4.5 line system of my own. But first I need something more important. Should all the leaders be equal length? I have some really nice leader material and quite a bit of it too but I still don't want to waste it. I am trying to make a through the bar rotating 4.5 line safety. Not sure it will work. Gonna test it on fence post or something heavy in back yard. I am gonna make it work somehow and it will be awesome

Well, all your lines need to be the same length so if your leaders aren't the same length your lines can't be either. If you're dealing with a standard 4 line lineset (not meant to be flown off a three line bar that spits to a Y) then your leaders need to be the same length or you have to offset your fly lines accordingly.

When the bar is all the way in should they be even?
What is the difference in line lengths when you use a y line.

When the bar is pulled all the way in to the chicken loop and the trim strap is all the way out (full power) the lines at the kite end should be the same length. If all four lines are the same length than the leader lines should also be the same length at this point. If your lines are of different lengths then you build the offset into your leaders from the bar in, trim out, lines equal at the kite end perspective.

If you use a Y line for your powers (so 3.5 line setup in your case) then the fly lines can be as short or as long as you want depending upon how long you make the | line of your Y. But they need to be equal at the kite end per the explanation above.

Plan on the finished bar to be about 1 meter long fully powered up as B-roc describes. That's what I generally got when I had a two-one mechanical advantage in the trim. If you have a higher mechanical advantage in the trim the bar has to be longer.

I know I'm probably light on details. Ask questions if this is confusing.

Philip