The 4gb pi4 is a considerable step up in RAM, if thats the limitation. The CPU improvements might not make material difference.
Once you have bought the pi, a case with heatsink (or fan), a new PSU you still wont be approaching the cost of a new NUC or the fanless ?BRIX equivalent. The cheap atom fanless PCs can be ok, but the price will be higher and only the most expensive of them have more than 4gb RAM.
I suppose the alternative is if you have any other NAS or 24/7 machine running. For example, I use a fraction of my Plex server.