A plain block of 4 does not seem to add any value over a single stamp. An inscription block of 4 does seem to add a tiny bit of value, though as you say the market for UN stamps in general including FDCs is in a sorry state.
If you look at completed auctions on
ebay - the ones that actually sold as shown by the prices in green type - most United Nations FDCs do not sell at all. That small percentage that does sell seems to be cases where a collector needs something in particular to fill in a collection and is willing to pay the cost of handling and convenience for the fun of completing their collection rather than people looking to buy because of perceived present or future value. When UN FDCs of any age sell, including the recent ones of the past 10 to 20 years, they tend to go for around $1 to $2 each, though there are many examples where special collection books of them or just plain kiloware of them go for much less. I noted one recent auction where 5 pounds of UN FDCs in nice condition all in plastic sleeves went for a total of $9.75.
If you want to put in the time, cost, and effort to try to sell one by one, or even sets, I am not sure you would find much profit when considering how much more you could make in that amount of time in most any other kind of work.
Or, given the state of the lack of value for most every UN FDC, you might think about using your accumulation to form the core of a collection for yourself and start buying UN FDCs to fill out your collection. It would be perhaps one of the least costly hobbies you can find and the fun would enormous.