how to get the 4 pieces

The quarter keys? That's going to take a while to do...it's pretty much the entire game. Basically, all you need to do is note the kingdoms marked on your World Map that have keys over them. These kingdoms are Quinn, Faiara, Orc, and Dragkthor. The one from Quinn kingdom(in case you didn't play Lord of Twilight) is now in Witchwood, thanks to Heptitus. Really, the gameplay for Gates isn't strict. There are places you should go first to avoid fighting monsters that are too strong, but you'll be able to see which places you should go first. I really can't tell you where you should go(except that it'd be best not to go to the Orc Kingdom until you have some nicer spells for Lydia, otherwise those stone frogs will be too tough!). But that's really the only one that needs to wait. When I first played, I went to Venwood(the city on the strange, labyrinth-style island) first, so you can do whatever you want. To get the keys, you'll need to perform a variety of different tasks first(example, in Dragkthor, you'll need to battle the key keeper to get the key).
I don't really want to spoil the entire game for you by giving you a blow-by-blow on how to get the keys. If I misunderstood your question, I'll try to answer it better.
I hope this is something of what you're looking for! Good luck!