B5 pretty much stands on its own, as do most of the Mongoose d20 (non-logoed) games nowadays. A change to 4.0 would likely not affect them, as many players buy them for the licence and not because they are d20-derived.
The stuff that would be far more likely to get hit are things such as the Complete Campaigns and Quintessentials. Mongoose are lucky enough to have distanced themselves from the D&D core rulebook long enough ago to give them some good insulation against any changes in that area.
IMO, the companies that need to get their contingency plans into gear are those such as Necromancer Games who pretty much rely on creating d20 "support" products. Many of the "d20 support" companies (although not Necromancer specifically to the best of my knowledge) seem to be in denial over the whole issue of 4.0, assuming that enough of their current customers will stay with 3.5. I think this is somewhat of a mistake on their part, because even if they do, you can't live off an existing customer base forever. You have to have a constant cycle of new customers coming in to replace the old ones that decide they've had enough of RPGs in their life - a large proportion of RPG customers walk into their local store and buy a few books for maybe 3-5 years, then give up on the whole thing.
A non-OGL 4.0 would cause the number of 3.5/d20 players to dwindle. Seeing as the d20 market is already swamped with too many products, there would be a shrinking customer base and far too many publishers trying to milk it.
Looking back at the move to removing d20 logos from Mongoose books and making them standalone games, you could even think that maybe someone thought about all of this long ago, and that Mongoose are now safely ahead of the game here.