Of course it's not about me. I realize that. It's about dss. He wants me there of course and so does my DP- his father, I also want to be there and share the special moment of him going into school
At four years old, he cannot understand the significance of the event. It will be later in his life, when he looks back and remembers these events, that he may well resent the fact that his parents allowed/insisted that he share those moments with someone who may, or may not, have continued to be an important and significant part of his life into adulthood. You can't undo those memories, they will be with him forever - and I don't think any step-parent has the right to be a part of them.
I have accepted that being a step-mum means that you do miss out on the traditional "special moments" that parents share with their DC's. I don't believe that it is the right thing to do to insist that you share them, or even accept that the DSC may want you to - you are the adult, and more able to recognise the long term implications on the DC, on their relationship with you, and on their relationship with their parents.
But you can create your own memories with your DSC - which are far more likely to be appreciated and valued as they grow up. Sharing the special moments they have with their parents is risky and selfish, imo.
DP is also dss's parent and has the right to make choices about who is an isn't involved in his life as much as she does.
This is exactly what my ex is saying about DD and his stbDW's involvement - what he won't acknowledge is that his choice is driven by his own wants, needs and desires. DD doesn't need her SM at this sort of event, her Dad needs his DW there - just as your DSS doesn't need you there on his first day of school, it is your DP's (and possibly your own) needs that you are fulfilling.