I put them on my 40,they are quite flexable.I used Skyjackers nitro shock part #N8023 and N8060.Made shackles 2" longer than stock which gives me a total of 5" of lift.
The only problem I had with Downey was that they had my zip code wrong and it took 6 weeks to recieve my springs.
I've never really heard much of anything good about downey, other than their products, which in some cases are good, but I've heard a lot about people having a lot of trouble getting what they ordered etc.
For a 4" lift, Skjacker is the way to go. Doesn't implode your wallet like OME, and still rides + flexes well.
p.s.-There are about a billion threads on this topic, might want to read some of those