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It can happen in the burn process, thatswhy some burn programs offer an option to check the data that was written. But there are several more things to think about:
1. Using different drives to play back optical media. Each drive is somewhat unique, especially regarding error correction (yes there is error detection and error correction implemented in the DVD sectors - the industry knew there is a need for it because bit errors are more likely). So what works error free on one drive might not work error free on another (Usually the media plays back best on the drive it was created on, provided that the drive did not change over time, i.e. loosing power on the laser diode or dirt and dust related issues).
2. DVD+/-R data layers are very thin, and since they need to be writable (with laser light) they are somewhat damageable to light (reading out a DVD often -with the drives laser- also means that data becoming errornous is more likely)
Imagine a fresh DVD+/-R as zero filled media. The laser burns some 1's between the many zero's (resulting in a bit-sequence). A 1 is basically burining a hole into a thin layer, so light will reflect differently on this position. However if this layer is exposed to intensive light sources over longer time period, it is possible that additional holes are "burned" into this layer, thereby altering the original bitstream. The DVD might still be readable, but contains some biterrors now. A few of the errors can be covered by the error detection/correction methods build in, but if the errors become too many, correction might be impossible.
Real DVD with aluminium layers are pressed with a so called master. The master is basically the negativ of the DVD (in terms of data). By pressing it with high pressure on a thin aluminium foil it will leave an imprint behind. This imprint has nearly the same optical capabilities like the holes in the DVD+/-R media. However such imprint is not damageable to light since it was pressed by mechanical forces not written with light. So a normal DVD should have a much longer lifetime than DVD+/-R media.
3. Copying things often is also not good regarding data consistency. (though lets say a period of 4 years is not often imo, and quite right, because if the period lasts too long, the data might already be altered too much or the devices to read it out behave differently). (There exists a nice analogy in biology regarding copying DNA and its impact on ageing)
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