Why do you change the bend when changing the rein across the diagonal - it's a straight line therefore the horse and rider should be straight, no bend. The canter across the diagonal is correctly termed as true canter, and it tests the horse and rider's straightness in the canter as it should be ridden as a straight line (and we all know how difficult straightness in canter can be!
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As for the tests, I would say that IMO P4 is the "better" test for a novice rider who thinks she or he may forget the test (it's just so symmetrical!), but P10 offers the rider more of a chance to show the horse off without all those pesky straight lines in front of the judge! If you don't like centre lines, definately don't do P4!
Of course if you decide you like P10 better but are worried about forgetting it, check if you are allowed to have it called for you (you should be).
Finally, the "official" (recognised by British Dressage) dressage tests run from prelim, to novice, elementary, medium, advanced medium, then advanced. Then the FEI tests start but we mere mortals don't talk about those
Some show centres and clubs design their own tests, some have walk and trot ones which are prelim standard minus the canter (usually). Tests are technically easier in the lower numbers, but it's not always the case... I hate P13 and P14 as they are just too busy and have things like give and re-take the reins, and I think the nicest prelim test is the highest (current) number, P18. N20 is nasty but N21 is nice. It takes a bit of experience to find which tests you like and your horse likes, but for a first go it really doesn't matter. Pick a test and go and have lots of fun!