Would externals be more prone to shaking and shock? Depends. First off, the drives are secured rather well inside their enclosures that they are sold in. Second, the drives may be spinning disks, but they aren't going to break from a few shakes. Third, if you keep the drive securely placed on a solid flat surface or put away in a secure location, there won't be shocks that you'll need to be careful for. By the way, if a computer virus or a power disaster wipes out your PC's data, all your data on the 2X2/4 TB drives are gone as well. So much for backups eh?
Also, in terms of risk, consider this simplified explanation. The numbers are selected purely for simple mathematical explanations, and do not reflect any reality.
Consider the probability of a drive failure to be 30% for 2TB drives and 40% for 4TB drives.
The Probability (P) of losing all data in the 4TB setup is 40%
The P of losing all data in the 2TB setup is 9%
Thus, the guys above recommended the 2TB setup.
However, the P of losing ANY data in the 2TB setup is 51%
While the P of losing ANY data in the 4TB setup is still 40%
In other words, both setups have their own strong points and their drawbacks.