One challenge this empire is going to face is that if you do Alexander->Ptolemy->Alexander's-son->"A grandson of Alexander, right?" is what rulers do with eligible candidates for the throne that were not actually chosen is going to come up for everyone after Alexander.
By saying Ptolemy becomes king after Alexander, and saying this is accepted, what we have as as a consequence is that "potential candidates" in the time of Alexander IV don't have to be descendants of the previous ruler, or even of the Argead dynasty. I don't mean that this means the empire collapses no matter what - Rome managed to endure for centuries despite the same issue as relates to that "legitimate ruler" and "de facto ruler" are basically the same thing, but it is a thing that would shape how things develop from there.
You might see it become in practice all but hereditary primogeniture (Rome, eventually) eventually, you might see some other way to resolve this (though with that the ruler doesn't even have to be of the "right dynasty", something comparable to how the Ottomans avoided civil war after the death of a sultan isn't even feasible) - but that issue, and the attempts to work around it, are going to profoundly impact the empire.
Not comfortably, judging by the issues faced by - most recently - Amyntas III, Alexander II, and Perdiccas III.