A lot of people here are speaking going on their own circumstances rather than the OP's. If you had your children when you were 22, 24 & 26, then of course you wouldn't want a fourth child at 45. OP is considering her second child when her first will only be 5 or 6, though.
As for being mistaken for their grandparent, there's another current thread dealing with this very issue. Considering that the 'normal' age-range for having children is 25-30 years, that's always going to be a potential factor. In some areas, 20 is considered old to be having your first baby and in others, 35 is considered young. A lot of it depends on your own circumstances, health, wealth and outlook on life.
From what she initially wrote, I'm guessing that OP isn't short of money and is likely to be in a position to be retiring comfortably relatively young. When/if her second were to be going to university, she would be in a good financial position to support him/her and then be able to be a work-free, well-off grandma and able to help out then when they might start thinking about having their own children.
Whilst it might be nice to be around until your children are 70 and to see your GC and GGC, it's by no means the experience that many people have. You have to live and enjoy the life you have, not the life you potentially could have had. My parents had my sister when they were 30 and me when they were 34. They didn't live to see me get anywhere near 25 or her 30 and they didn't get to meet their first GC, even though my sister was only 28 when she became a mum.
Ideally, our parents would have been very rich and been able to buy us everything we could have possibly wanted, but they weren't - they were on the poorer side of average, but it doesn't mean that we had a bad childhood or wish we'd never been born.
As for calling people (well, women) selfish for having a child at 45, you have to think if the child would agree with that that to the extent that, when old enough to consider it properly, they would have preferred never to have been born in the first place. The vast majority of people, from the whole gamut of life circumstances, don't end up thinking that way.
To look at an extreme case, Sue Radford had not long been a teenager when her first child was born. She's now 44 and expecting her 22nd child (very sadly, one of whom hasn't survived). Some would say she was selfish for having her first so young (granted, it's very debatable as to how much agency and ability to consent she had then), some say that she's selfish for having so many and some say that she's selfish for having any number baby at 44/45.
To be honest, you could easily say that ALL planned pregnancies are chosen for selfish reasons. Most people don't have a baby because of what they think they could offer the baby but because they want to be a parent.
I think you just have to live your own life and make your own choices as best you can and play the hand you're given. Hindsight is always a great thing, but no matter how much you may look back and wish you'd done something when you were much younger, you can't actually go back and change that at all.
Personally, I don't think 45 is too old to have a baby, if your circumstances are otherwise good. Maybe we should just leave it to nature and say that, if you're able to get pregnant (at the upper end, in your 40s, at least, as opposed to the lower end, when you may be 9 or 10), then it's not a terrible thing.
It's definitely not categorically a black and white yes or no matter.