I would say for starting out get a 2 stroke.
Reasons why: The cheapest 4 stroke is about twice as expensive as a 2 stroke. For the price difference you can buy fuel and spare props instead and fly all summer. A 4 stroke that would work well for a trainer would be too small for the average sport plane. An average sport plane should have a 90 size for a 4 stroke which would be way overkill for a trainer. Do you really want to big bucks for a trainer size 4 stroke engine and then have to spend it over again plus about $20.00 for a 90 size for your next plane?? Most 2 strokes made currently are ABC type engines which are fast and easy to break in, inexpensive(relatively ) and reliable. A lot of the better 4 strokes are ringed engines that have to be broken carefully and slowly and they can take 2-3 gallons of fuel before your done vs. 3 tanks of fuel for a 2 stroke. Most instructors will be familiar with most of the 2 strokes and that will save you time and trouble. When you are ready for your 2nd plane then you will have more of an idea of what kind of flying you want to do. 2 stroke engines are cheap enough that it won't hurt to get one to start out with worry about 4 stroke power later.