Do SSD's fail suddenly or is it usually a gradual process ? I don't think there is an exact answer to this one. They can fail both ways. I'm certain some have turned on their machines and the drive was working fine before shutting the computer down, and on boot up the drive just isn't detected at all and never works again. I've had one that was fine for 2 or 3 months, then Windows would freeze for a second or two every now and then, but would recover, then Windows started freezing for 5 or 10 seconds and sometimes recovered, then the drive just wouldn't fully load Windows, then it wasn't being detected as a drive.
Do hardware RAID cards fail suddenly or gradually also? This could probably go both ways as well, but I would lean toward them failing suddenly more so then over time. The card itself may not be detected as all if it died, or it may be semi-functioning but dropping drives out of the array etc. A lot of times people will buy two raid cards when running a setup with important data to ensure they can just swap the cards out if the controller does die.
4 SSD's is Raid 0 is a bit crazy to me, but it sounds like you are well aware of that and prepared to work with any issues that come up.