What's the difference between VCF spec versions 4.1 and 4.2? What are the key differences between VCF versions 4.1 and 4.2?
 It looks like v4.3 contains a changelog (specs available here) but earlier specifications do not. 

This biostar post points out one difference: the introduction of Number=R for fields with one value per allele including REF — can anyone enumerate the other changes between these two versions?
This is easy to check, you can download both specs in .tex format and do diff.

Changes to the v4.2 compared to v4.1:


Information field format: adding source and version as recommended fields.
INFO field can have one value for each possible allele (code R).
For all of the ##INFO, ##FORMAT, ##FILTER, and ##ALT metainformation, extra fields can be included after the default fields.
Alternate base (ALT) can include *: missing due to a upstream deletion.
Quality scores, a sentence removed: High QUAL scores indicate high confidence calls. Although traditionally people use integer phred scores, this field is permitted to be a floating point to enable higher resolution for low confidence calls if desired.
Examples changed a bit.