The effect of a base power 4 counter or base toughness 4 counter overrides any existing effects that set that creature’s base power or toughness, as appropriate. Any existing effects or counters that raise, lower, or switch a creature’s power and/or toughness continue to apply to the creature’s new base power or toughness. Any base power- or toughness-setting effects that start to apply after the counter is placed on the creature will overwrite the counter’s effect. (For the rules-savvy, yes, this applies in layer 7b.)

Playtest cards aren’t legal for play in any tournament format other than Mystery Booster Limited formats. On the other hand, we expect they will spice up a wide variety of non-tournament games (as long as everyone’s on the same page about using them!).

Playtest cards use a modified version of game symbols, such as {T} and {W}. These modified symbols should be treated as the standard symbols during play.

For many playtest cards, you’ll need to make a generous assumption that basic game rules would be updated to allow them to work. The Mystery Booster 2 Playtest Card Notes section (reproduced here in individual Gatherer rulings) provides guidance for fitting these cards into the existing rules structure.