Obviously, there'd be no Hundred Years War
Would it be *this* obvious? There's a tendency to call the Capetian-Plantagenêt conflicts of the
XII-XIII centuries the "First Hundred Years War", and it makes some sense : it after all covers all the same different conflicts with periods of truce and opposes two clearly identified royal powers.
Giving the conflicting interests that were present in the XIVth century, the network of alliances, you'd certainly end with an important conflict there. Probably "not the one we knew" as you said, but really, if there's a period and a place where it was going to go badly, it would be there.
However, would there be a shadow over his legitimacy because of his mother's involvement in the Tour de Nesle Affair?
*Sigh*
While the whole affair shouldn't overlooked, its importance had been inflated way beyond sense, with both the gravelous legend about executed lovers but as well some...bias (I won't dwell about what I think of Britto-American historiographical stance on HYW because it would derail the thread too much, but, yeah, it's a thing)
One have to remember that intimacy in this era is an empty word, even for nobility and even more for Capetians ( known for a policy of court simplicity if not austerity) : it's why the affair went public as it did.
They were ways to prevent unwanted pregnancies, as such would have been obvious for anyone : a future queen being pregnant was bound to be under heavy scrutiny (one could argue it's one of the reason Blanche acted as such eventually) and surviving sons may simply have prevented her being part of the affair.
Eventually, even if it happened, one shouldn't forget the shitload of scandals and affairs with queens in France in this era, that never really went more than rumors and mumblings.
Assuming Charles IV still dies in 1328, Philip would be 14 years old, the age of majority for Kings of France.
Age of majority wasn't fixed in France until
Charles V.
At this point, it doesn't mean anything by itself : I'd actually expect some regency from Philippe de Valois, for at least some years.
It doesn't looks really obvious.
Marriages doesn't just happen CKII-way, there's actual reasons safe "royalty" and age, namely geopolitical concerns : remember that last direct Capetians married into french nobility, and it might be the case depending of the priorities.