Which four stroke ?
IMHO Saitos or O.S are the way to go, if you can afford it. If the plane is tail heavy go with the O.S ,they weigh more than a Saito.
If your on a budget , Magnum and ASP are much cheaper , and run very well . The manufacture recommends a synthetic / castor blend , because they run a steel sleeve . They usually do weigh more.
YS engines are awesome, but they are probably not a good choice for your first 4 stroke unless your work background has to do with mechanics . The fuel delivery system is more complicated and they are a little spendy.
I fly Saito ,O.S. Magnum and ASP. 4 strokes , and very happy with all of them. Out of all 4 brands , my Saitos have more power are lighter, The O.S . Magnum and ASPs seem to have about the same power , but the O.S. does have nicer HS and LS needles , but they are all good engines.
Once broke in , they don`t take much fiddlling with.
Hope this helps
Robbie