One of our rescue Weim's has a class 4 (we think she is probably 12ish). She is currently taking Vetmedin (expensive and hard to find), and we just added Furosemide.
She was diagnosed and put on the meds about 6 months ago, and is on the 6 month check up regimen. We are going to a new cardiologist now that said once we run out of the Vetmedin, that there is something that works equally well, and a fraction of the cost. I'll see if I can find what that new med is.....
As far as the effects of the murmur, you really wouldn't know she has it. She is still able to go on long walks with us, and can play.