Frankly the "value" of the car is what someone is willing to pay. Someone may just be dying to have a Pinto and pay overprice because they had to have it - today! Other times there is a car that has a high value be cause it is exclusively optioned, has exceptionally low miles etc..
The problem with these buyers and/or cars is that once they set a basis to draw the money they do EVERYONE thinks their car is worth near that amount. I troll Craigslist often and I see the same overpriced junk month after month. Typically it is a long time stranded car, disassembled engine, flat tires, faded paint, rotted interior, lost title, in a far off location etc.. Somehow the seller thinks this is worth 75%-100% of a decent car. The other scenario is a decent car that the seller thinks is just a wee-bit short of a fabulous car. The car typically has almost the price of the fabulous car even though the true value is more like 50%-75% of that price.
Smog test ramifications can also devalue a car. And, in the Pinto world the '73-'74 break on the bumpers is likely a consideration. All that said if the image you provided is a true representation of the whole car (it does look very clean) I would not call $4,000 unreasonable. As a seller I'd hope to get that one buyer who would be willing to spend $5,000-$6,000 for it and as a buyer I'd hope you were desperate and take $2,500 for it.
Every buyer has their own motives for a purchase. As a '74 I'd hope this Pinto has the 2.3 and since California doesn't smog until '76 - and up, I'd feel free to modify it. Lastly the car has the possibility to retro modify the bumpers back to the early Pinto appearance.