Every seller of cloud storage services will tell you that their service is "secure". But what they mean by that is something fundamentally different from what we mean. What they mean by "secure" is that after you've given them the power to read and modify your data, they try really hard not to let this power be abused. This turns out to be difficult! Bugs, misconfigurations, or operator error can accidentally expose your data to another customer or to the public, or can corrupt your data. Criminals routinely gain illicit access to corporate servers. Even more insidious is the fact that the employees themselves sometimes violate customer privacy out of carelessness, avarice, or mere curiousity. The most conscientious of these service providers spend considerable effort and expense trying to mitigate these risks. However, some privacy breach is inevitable--if by nothing else, then by government order via programs like PRISM.
What we mean by "security" is something different. The service provider never has the ability to read or modify your data in the first place: never. If you use S4, then all of the threats described above are non-issues to you. Not only is it easy and inexpensive for the service provider to maintain the security of your data, but in fact they couldn't violate its security if they tried. This is what we call provider-independent security.
This guarantee is integrated naturally into S4 (and LAFS, on which S4 is based) and and doesn't require you to perform a manual pre-encryption step or cumbersome key management. After all, having to do cumbersome manual operations when storing or accessing your data would nullify one of the primary benefits of using cloud storage in the first place: convenience.