A few factors are to be considered. His kids are older and the norm is that the older the child, the more you spend because what they really want is usually more expensive.
Saying that, if there is a big difference in age, the elder might not care that the younger ones are getting three books whilst they get one. Closer in age, with youngest getting the latest phone and an iPad, whilst the older are getting an older phone version might be an issue.
And then comes in what they get with their mum. If they get the phone at their mum and dad gets them the iPad, they might not care.
There is also their personalities. My kids never cared if one got more than the other and same with their step siblings. They were not the envious type or children who compare, so it was less an issue. For children who do gold a strong sense of justice, it might be very hurtful they are getting less, even if all together it is the same.
Lastly, the experience. If they've open their presents, and they then gave to spend the next half hour watching the others open them with the adults building the fun and excitement, they might feel inadequate.
Consider all those things with your OH to decide all in all how you can ensure Xmas is a nice experience for all 4 children.