You may want to look into a SAN solution. I worked with a client that had large data needs. They were adding TB's of new user-supplied data every month. Due to budget, we opted for cheap SATA disks in RAID 5 and crammed as many as we could into a 2U server. This worked for a while but as they kept growing performance issues, scalability and management issues arose.
Solutions such as EqualLogic, NetApp and others provide very high scalability and good management tools -- they have price tags to match.
You've not specified how much bandwidth you expect to use, but looking into cloud based solutions that also provide delivery services could be another option, especially if you looking for a pay as you go solution.
We have access to Internap's CDN via one of our partners. We can store data on their systems and then have it delivered via their CDN. I've never inquired as to 40 TB of storage but would not think it to be an issue.
If you do build 4 10TB systems, you will need to flush out these questions:
1) How many front end server nodes will connect to them?
2) How often will the files be accessed?
3) How do you plan to backup the data?
4) What connection method will you use NFS, GFS, iSCSI?
5) How will traffic/storage requirements change over time?
All of these are very important considerations into picking a technology. While building 4 10TB systems may be easy and relatively cheap, if they fail to handle the service loads in 3-5 months, then your money will have been wasted.
Often with media sharing sites (if that's what this is), some files are more popular than others. 10-20% of your total content may account for >80% of your total traffic. In this case, you may want to consider "fast" and "slow" storage systems. Keep the popular stuff on the fast system and archive off the slower system.