uhh, your first two sentences are:
喫茶店に行ったら、鈴村さんに出会ったかもしれない 。
喫茶店に行ったら、鈴村さんに出会ったかもしれな い。
They are the same, I think you forgot to change the second one?
3) 喫茶店へ行った時、鈴木さんに出会った。
and 4) 喫茶店に行ったら、鈴木さんに出会った。
There are two differences, one being the use of the particles へ and に, which both denote direction (with slightly different nuances), and 行った時 vs. 行ったら
while both へ and に imply the direction of the verb 行く, へ can also mean to head in that direction, and not necessarily going straight there.
行った時 is pretty straightforward: the time [subject] went to/towards [destination]
行ったら can be used as a conditional (with a high level of probability), but in this context, it refers to an event immediately following the completion of the action.
Had you 喫茶店に行ったら、鈴木さんに出会う, it would mean "If I/he/you went to the cafe, i/he/you would run into Suzuki-san" (although if you were planning on meeting Suzuki-san, the term 出会う is kind of moot).
So, to translate the two sentences, I would go with:
喫茶店へ行った時、鈴木さんに出会った。
When I went to the cafe I ran into Suzuki-san.
and possibly: I had left for the cafe when I ran into Suzuki-san.
or: I was headed towards the cafe when I ran into Suzuki-san.
喫茶店に行ったら、鈴木さんに出会った。
When I went to the cafe I ran into Suzuki-san (implying that Suzuki-san was there when you arrived)
So, they're very close in meaning, but carry slightly different nuances. At least in my interpretation. Language is wonderfully difficult to fully grasp!