I've seen a few cards that say R (4+): effect. What does the 4+ mean?

It means that as an additional cost to play the cards you must make a control check against difficulty 4.

You can commit foundations to pass this check as normal (since it's vs. a difficulty).

Stamp for Aslum!

I see. Is it affected by the number of cards in your card pool?

No, progressive difficulty is not applied.

Thanks guys.

Question answered. Topic closed.