I want to know the difference between the next sentences...
1. Watching TV is fun.
2. To watch TV is fun.
3. It is fun watching TV.
4. It is fun to watch TV.
Words ending in -ing express actualized events. Either the speaker has experienced it or is doing it. to-infinitive verbs express unactualized events. Either the speaker hasn't experienced the event or has but is speaking of the event as a impersonal event, something general. For example,
I like to swim. (general event)
I like swimming. (I have experienced it).