Welcome to Flyertalk,
starnatty.
And welcome to the small group of CSA flyers on these boards.
Your FFP decision is a total no-brainer. Join the CSA OK+ programme.
4 flights a week mean that in 4 weeks' time you will make silver. From the fifth week onwards, you'll have lounge access in Prague and a decent chance at sampling the excellent business class product (complimentary space-available upgrades). Another 4 weeks, and you'll have Gold, giving you lounge access all across Skyteam and virtually guaranteeing your upgrades on Czech planes.
Enjoy the ride, and please avoid taking many flights from/to MAN, as I need the upgrades.