It's hard isn't it. I've just started and I'm 36 (!) and pregnant for the first time. I hope to have 2 before I'm 40 (fingers and toes crossed). Congrats on your family so far! I guess there are two schools of thought - it's your life, live it as you want to with your family or that it's potentially a bit selfish due to age.
I genuinely don't think there is any point asking random people that you've never even met what they think...it's a decision that you and your husband and your kids (!) need to make together. It's you as a family unit that counts, not anybody else's opinion. How do you feel as a mum just now? How do your older kids feel about it? Have you spoken to them? Would they support you and your husband if you had any ill health etc.? The thing is anyone of us could come down with bad health tomorrow, irrespective of age, so again - is this even a question worth asking?
I'm just posing questions to help you think about it. I've not got an opinion either way, I've a friend who was 45 when she had her first child and she dotes on her like nothing else & is the best mum. Sit and work it out with your whole family - not with strangers! I wish you all the best x