You should be able to use the Exchange server to send bulk email, or use the Edge transport server as a relay if you have applications sending mail directly https://learn.microsoft.com/en-us/exchange/architecture/edge-transport-servers/configure-without-edg... Depending on the way you do it, your Exchange infrastructure may be tagged by various message hygiene systems for sending bulk email, so you'll need to manage it correctly otherwise it may affect your normal business messages
4 public IPs could be NAT'd to a single server, incoming, but not being a network person, I'm not sure how you'd configure a single server to send out and use 4 different public IPs
