Hey there,
it depends mainly on the company's long-term strategy. If the company is going to have more products in the future (in addition to the mentioned 4), it wouldn't make no sense to make for each product special domain. Therefore, users could be a little bit confused and you would unnecessarily split the domain authority. Better to have everything under one roof.
In my opinion, it also depends a little bit on length of the domain name. If the domain of the new product is easy to remember, short and makes sense, then I wouldn't change it now. You risk the effort of the previous 6 months.
So from the view of long-term strategy, I would change - however, not to subdomain but to subfolder (domain.com/productname). Regarding the effort of the previous 6 months - I would review the amount of back links and how hard would it be to eventually manage them.
That's just my opinion, hope it helps a bit. Martin