You've got the basic idea right. The trim setting on the front lines adjusts the angle of attack (AoA) of the kite to the wind. Full power is with the front lines extended as far as possible (max AoA), full depower is with them shortened as much as possible (min AoA).
Here's a good video about general bar tuning:
Here's one specific to Cabrinha, but has some good general info in it too--I like the tip about using your leash as the anchor point.:
Yours is new, so it should be in tune; but it's best to check anyway. You can ignore the differences between that bar and yours: pulley bars are a relic of a bygone era and only a few brands have below the bar depower.
Industry standard these days is to have the 4 lines of equal length with the trim set to full power and the bar against the chickenloop. If your new bar doesn't conform to this at all, then check online; there may still be a few kites on the market with mismatched lines. You'll definitely need to make some pigtails for the trainer kite if this is the case.
If your bar conforms to the standard, then there should be enough range in the trim adjustment and depower throw of the bar to at least get the kite in the air. To launch a 4-line foil with a bar, put sand or weights over the trailing edge of the kite, walk back to the bar and hook into the chicken loop (CL). Put the bar as far away from the CL as possible and give a tug on the front lines above the bar as you take a few steps back. Provided there's enough wind, it should come off the ground quickly and easily.
If you can't get the kite in the air like this, or if its doing so slowly with a ton of pull, there's probably too much tension in the rear lines. The first thing to try is to decrease the AoA by shortening the front lines as much as possible with the trim adjustment and try to launch again. If it's *still* fighting you to get off the ground, you'll need to add some pigtails to the rear lines
The opposite scenario is the kite comes rocketing off the ground, and goes flying over your head before falling nose-first. This is called 'luffing' (or a 'hindenburg'*) and happens when there isn't enough AoA to keep the kite from over-flying the window. In this case you need to lengthen the fronts by setting the trim to full power. With the kite in the air, if the rear lines are still slack with the bar against the CL, you'll need to add pigtails to the front lines to increase the AoA even further.
TL,DR: Too much AoA = too much tension on the rear lines: The kite stalls more easily or doesn't fly at all. Too little AoA = overly slack rear lines: reduces steering control and lets the kite over-fly the window.
Hopefully that's got you primed on tuning your kite and bar. It will feel more intuitive once you've got the kite in the air and can play with the trim adjustment. It's another thing a 4-line kite can teach you that a 2-line cannot, and will pay dividends down the road when it comes to tuning your full size kite.
*Hindenburgs are bad news. If you're lucky, the kite will fall to the side, away from the lines and you'll be able to re-launch the kite. If you're unlucky, it will fall in between the lines and then get pushed 'inside-out' by the wind--this is called an 'inversion'. Sometimes you can fly the kite back to the beach inverted, other times a bridle can get caught on a wingtip and you'll be unable to clear it. You don't need to worry about this for now, but ask your instructor. You'll save yourself some swimming if you know how to avoid them in the first place, and must be prepared to deal with it when it happens to you--because it will.
P.S. Your bar and lines will probably have 'kook-proof' pigtails on the ends (with loops for the steering lines, and knots for the power lines--I think that's standard these days). Most regular 4-line foil kites I've seen have simple knots on the ends of the bridle to match loops on the lines. Since it is meant for use with a standard kitesurfing bar, your kite might have the loops on the front attachment points, and knots for the rears to correspond with the pigtails of your lines. If it's all knots on the bridle you might have to remove the pigtails and store them in a safe place (and if needed you can use the ones with loops on both ends to tune). Make sure you don't lose them, or get the fronts mixed with the rears when moving up to your full size kite, it can prevent your safety system from working with disastrous consequences.