With four drives, your configuration options are as follows:
1) Two mirrors, one mirror each into a pool. Result: two pools.
2) Two mirrors, stripe both mirrors onto one pool. Result: one pool.
3) One RaidZ2 with four disks. Result: one pool.
With the first option, you can lose one disk from a mirror without losing data. If you lose two disks from the same mirror, then that pool is gone. The other pool is unaffected.
With the second option, you can lose one disk from each mirror without losing data. If you lose two disks from the same mirror, then the entire pool is lost.
With the third option, you can lose any two disks without losing data.
What is best? Well, it depends on your priorities. The write performance for a mirrored pair will be better than RaidZ2. Option 2, above, will have better write performance than Option 3, but Option 3 will be somewhat "safer" in that any two disks can fail before data is at risk.
For a media server or backup system where write performance doesn't need to be optimized, I would consider RaidZ2 to be the better option. For a system hosting VM's, or for which write performance is critical, then mirrors might be a better choice. For a system that does both, one could set up a RaidZ2 pool for data, and a mirror for the VM's.