CheekyTiki: is this a hard series of games?
It's a difficult question to answer because it's so subjective. Hard compared to what?
I've played 1, 2, and 4, and I would say 2 is harder than 1 is harder than 4.
2 is also quite different from 1 and 4 because it's about pirates. 4 is like a vastly updated version of 1 (but not necessarily better in all ways).
The difficulty comes mostly from the different campaign scenarios. It gets tricky when you have to achieve goal
x within a time limit, or with limited resources. The scenarios guide you through the different game mechanics step-by-step. For example, you might start out with a scenario about food production, and that will be your focus. Later you'll be focusing on tourism or mining or law enforcement or whatever.
If you play sandbox mode I wouldn't say Tropico is all that hard. You encounter problems, of course, but once you understand the methods of solving those problems, you can usually figure out a way to deal with them.
Tropico is one of those games where losing can be fun. You can decide to play a crooked dictator who tries to steal as much of the nation's money as he can get away with before getting ousted from office.
Overall I would say the Tropico games have a moderate difficulty level. I don't know whether or not that helps you.
Personally I like the atmosphere and tone of 1 the best, I like 2 for its uniqueness, and I like the mechanics and ease-of-play of 4.