He is definitely doing more than most NRP that I know. Most have weekend daytime or every other weekend staying contact, with little if any mid week contact because of work/kids after school activity commitments
When I split with my ex, father of my DTS's 13 years' ago, I hoped we would still operate as "co-parents"-we had lived together for 12 years, and he was a good "hands on" parent to the DTS's then aged 10.
I couldn't have been more wrong! Ok, at first he was back living with his mother, but every Sunday from 1-7 was all he wanted. He initially helped with child care after school as I was in the last year of my BA, and didn't get back from Uni til 5-6pm some days, but he clearly resented the tie on his time and "juggling" his work for my benefit (as he saw it). In the end, my dad stepped in.
Things got worse. He soon met someone and ended up having 2 boys with her, got his own place 30 mins away, but never offered to have them in the holidays, take them away etc.
I expended a lot of energy trying to involve him in being part of the the secondary school selection process, being more hands on etc-for the DTS's sake. To no avail. He wanted regular contact but on his terms. Every Sunday was his "window". He was happy to forego attending Parents' Evenings, appointments etc.
He got the "fun, low key, access".....and I got all of the responsibility.
Back to you OP-sorry, that was a bit of an outpouring...it still rankles! YANBU to hope for more, but you may need to be content with what he is willing to do. He does see them quite a lot-and no, you don't have to be grateful for that, but rightly or wrongly the PWC will always bear more of the brunt. I have long learned that you can't change someone's feelings with the strength of your own. And, tbh, I wouldn't want my kids spending extended time with a parent that was doing so under duress.
Make the most of the free time you do have-those 4 days per month. Book a babysitter/call on friends or family for times when you need to be child free which fall inbetween the DC's staying contact. Don't give him the satisfaction of trying to appeal to his better nature/sense of fairness-cos it's likely not there!