The lack of understanding of grief on here is disturbing, though not surprising.
"He's being contradictory." Yes, probably. Grief gives rise to many contradictory emotions.
"It's been four years, for God's sake." It can take longer than that. The passing of time is not a measure of the passing of grief. And, as someone else pointed out, the processes of grief aren't linear.
"My dad died six years ago, and I don't do anything special on his birthday." Okay. Thanks for that.
"He should be concentrating on his family." He is. He's thinking of the family he's always been part of.
"Life is for the living." If we're going to trot out glib clichés, "In the midst of life, we are in death." Neither of those helps much, does it?
"He acted abominably, blowing up at you." Yes, maybe. But that's not relevant to the problem of what to do about this clash of priorities.
"He sounds very unstable." He probably is. Following the death of a close relative, my OH was unstable, unreachable, emotionally absent from me and the kids for about five years. Something had happened that was the emotional equivalent of an earthquake levelling the house. It sounds to me as if the OP's husband has held it all together quite well for most of the time.
OP says that her husband is an engaged, involved, caring husband and father. This 'selfish' kind of fixation, she says, is not typical of him So something's going on inside him that's important - so important that he wants to organise his family's holiday around it.
I think the family should recognise that and accommodate him, not only because that's what families do, but because - as the OP says - he is the husband and father he is because of his dad.