Sorry to disagree. Prelim 7 by a mile! You don't even have to canter a 20 metre cirlce and in Prelim 4 you have to trot and canter your cirlces between E and B. We always try and find a Prelim 7 test when taking out babies.
THink I agree with Galaxy here, p4 has two half 10m circles too, and centre lines can be tricky on a baby horse as strightness is very obvious to the judge when you are headed straight for them!
I had the choice of P4 or P7 last weekend, after riding through both I chose P7... miles easier IMO.
Depends on your horse though - I wanted a simple test for my horses first time and hoped the the amount of work around the outside would help settle him. P4 would be better if your horse is more easily distracted
I find P4 easier as my horse is quite tense and I find all the circles and things stop his getting too rushed in his canter. He's quite easily distracted so it keeps him listening.