Congratulations.
I had my 2 at 42 and 46. (They are not 8 and 4.) I do wish I had been younger when I had them (we were ttc for 10 years before DC1 and both were IVF babies much against the odds).
The downside is the financial drain at a time when you might otherwise be accumulating a bit of reserve for retirement - a bigger challenge in practice than I would have anticipated, although we were quite comfortable before DD was born, but not really that different from the financial challenges many younger parents have.
The upsides are many, however. Not least that we have been something wonderful at an unexpected time for me, DH and my own DM (whose other grandchildren are in their late teens/early 20s and who never expected to have another little one to dote on). Also, I know they keep me young (physically and mentally) and I have noticed that people tend to assume that I am younger than I actually am. I get knackered too sometimes, I won't deny it, and I think my life experience does give me a different approach to parenting than some younger parents but age is not the only definer in what kind of parent you are.
I do worry more sometimes about the prospect of being a mum to teenagers in my 60s but you will have the advantage over me of experience with your older two.
Yes, there are increased risks when you are an older mum but you will be closely monitored, and there are risks with all pregnancies. I personally think that the contributions from the two GPs above are extremely unhelpful but you can be sure that you will get plenty more comments like these from HCPs. Take the risks on board but keep them in perspective and do not allow yourself to panic or be rushed down avenues you don't want.
For what it is worth, we had no screening during either pregnancy - the risks of the CVS test were too high for us - but we did have a detailed anomaly scan at 20 weeks so that we could be forewarned of any potential problems.
My advice is to let yourself get used to the idea and then take each day as it comes. Definitely don't cross over any bridges till you come to them. Hoping that this is the start of a lovely new phase in your lives, as it has been in ours.