I would honestly leave that to individual posters (so that's a vote for d20 + 4e, I guess). While all "true" (SRD-based) d20 games theoretically have a common core of rules, in practice they tend to pick and choose which parts of that core they include. As a practical matter, very few "d20" questions will apply to ALL d20-based games. A question focusing on, say, D&D 3.x and Pathfinder will probably not apply at all to Mutants and Masterminds.
The result of this is that most questions tagged d20 will need to be scoped explicitely in the question description (either to specific games, or to games with a specific characteristic). Looking over the questions that have been tagged d20 so far, most of them restrict the scope of the question in some manner.
Regarding the hex grid question that brought this issue to light... While Brian's answer was littered with 4e-isms, and was overly negative to the point of being a little hyperbolic, the core of his points still applies.
When Brian references close bursts being weaker due to adjacency, this still applies. Eight surrounding squares dropping to six is a non-trivial difference for tactical games (which most d20 games are).
Likewise the notes about blast and burst targetting areas still apply. While 4e uses by far the most square area effects (3.x mostly uses pixelated circles), various d20 games DO still make use of squares. The most notable instances being monster sizes.