First of all, you sound wonderful OP and it’s fantastic how much concern you have for this little one. Thank goodness for you.
I think that your parents are being extremely naive and have no idea what they are letting themselves in for. I have twin 4 year olds who are both disabled so not like typical 4 year olds, but I am part of a Facebook group called Therapeutic Parenting and many of the people there are either adoptive or foster parents or are in a similar situation to your family.
When children are in this situation; it’s extremely traumatic for them - he is being taken away from the only life he knows and his only parent and moved to people he doesn’t know with no transition or gradual introduction. It’s also likely he’s already experienced trauma at some level if things have gotten to this stage. If his mum hasn’t been able to meet his needs (no judgement, clearly she is trying to do the right thing) then he may have past trauma, attachment issues and so on.
Often children with this history have behavioural issues, developmental issues etc. Many behave like much younger children, and especially when very distressed can almost regress back to being like a much younger child.
I don’t think any four year old can thrive in a house of adults where all of them are working full time and they are not in childcare, but even more so a child who is likely to need significant support particularly in the beginning. He doesn’t know any of you and he is likely to be confused and scared.
I think it’s imperative that both of your parents take time off initially so he can get to know you all. He needs reassurance and stability. If they are making the commitment to take him on they have to commit to doing what’s needed or things will get worse very quickly.
I would also consider that his development might be delayed when child proofing, buying toys etc.