It's always good to see people who'll buy the game after it's release because somehow Revolution needs to make profit with this game after all

All Broken Sword games don't tell a continuous story, each game has it's own story. You don't need to play any game before you can play any other game of the series. Revolution also said there is no real timeline of events, all games are happening "at the same time" somehow (obviously except for the first game, where Nico and George meet for the first time).
So you don't need to play BS4 in order to understand/enjoy BS5. But I would definitely go for it. I think the story and puzzles are better in BS4 than in BS3. On the other side, the presentation (atmosphere, characters [except for Brother Mark in BS4, one of my favourite character ever!], animations, facial expressions) is better in BS3.
My favourite BS is also BS2 (because it was my first Point&Click game ever and I played it on the PlayStation back in the day^^)