I think the whole point of "engine relevance" is that its relevant to current cars...as Ferrari are not trying to sell old cars from the 50s are they? Their current range of roadcars do not have Inline-4 engines, therefore, they cannot use the marketing potential of F1 in this sense.
Ok, they don't produce V6s either currently, but its at least a bit closer than an I4, and its something Ferrari is more likely to produce in future.
This is not about relevance to what a car manufacturer can produce (as most car manufacturers rarely if ever have anything physically to do with race engines) and its not about relevance through legacy or history. Its purely about relevance in marketing. They like to say "and our V8 engines power our F1 cars to victories, so you can feel the performance of victory" and that kind of stuff, trying to suggest buying a Ferrari roadcar is buying F1-developed technology (which in reality is only partially true, but as regards to engines, its not really).
Basically Ferrari is more worried about the image of inline-4 turbos for them, as they currently don't produce them in roadcars, it isn't the kind of image they are looking for. Same for Renault or any other manufacturer.
Perhaps in this respect a V6 is a better idea as it appeals to more car manufacturers who are likely to be interested in F1 marketing. While inline-4s are far more common, are Suzuki or Kia really going to stump up the cash for this kind of marketing? Probably not as much as Porsche, Mercedes, BMW, Ferrari, etc are. Your "average" car manufacturer isn't really targeting a performance market and so tends not to use much performance-based marketing such as F1 brand promotion. Then again, the global reach of F1 is huge and is very useful for any car manufacturer.
