How Rare Is This Square Colombian Coin?

No, not necessarily. The proof coin shown falls into the category of "collector coin" vs. "circulation coin." A mintage of 4,000 for a gold circulation coin would very likely be in demand and be valuable. The same can't always be said about collector coins.

Non-circulating collector coins generally have a smaller collector base vs. circulation pieces. So, the supply of many low mintage collector coins exceeds the demand and thus prevents them from reaching market values well in excess of their intrinsic value.

I don't know how large or robust the market in Columbia is for non-circulating gold collector coins. The NGC World Price Guide suggests the coin is worth its bullion value, but I can't independently verify this.

If you like the coin, haven't come across many, and it is being offered to you for 20% or less over its intrinsic bullion value, it is worth considering IMO. I just wouldn't expect to quickly turn around and sell it for a profit.

Best of luck!

No—shape doesn't determine value. I agree that it is likely primarily worth the spot gold price, but the novelty of it could make it a fun addition to your collection.

Thanks everyone for the replies. I will try to negotiate the price as close to spot as possible. Also to weigh it, measure the dimensions, and look at it with a loupe before purchasing to confirm 100% it's real. Cheers.