I suppose, in principle, a static ipv4 address does degrade security to a small degree - but not, IMHO, to the degree that you need to worry about it.
The big increase in risk is that, should a bad agent discover a weakness in security, they will be able to exploit it indefinitely (or at least until you fix the weakness) where as with a dynamic ip address, a change in WAN side ip address hides the issue again (at least until it is re-found).
It should also be noted that different ISP’s implement dynamic ip addresses in different ways. When I was with Virgin Media in the UK, on a couple of occaisions I retained the same ip address for more than a year. Now I am with BT, it seams that my ip address changes every time I reboot my router (and sometimes more frequently). I would contend, that the stable dynamic ip address that I had with VM does not offer a significant security advantage over a static ip address.
In either case, good security practices (both in the admin of the router and user practices) are most important.