If you don't mind I would like to give you my perspective as to the value of your B6100. To give you a little history of where I am coming from here, I have bought and sold (2) B6100 in the last 4 years. One a hydo 4WD and one a gear shift 4WD. The hydro was is tougher shape than the gear model and sold it for $2750. The gear shift was in really good shape with only 485 hrs on it. I bought it with a Kubota B40C tiller for $3500. I later traded the tiller in on a bigger one and then ended up selling the tractor for $3250.
The reason you are not getting anyone looking is because in my humble opinion, it is priced too high. If you were selling it with some attachments, then you may be able to get that kind of money for it. But not the tractor alone. Of course different parts of the country will bring higher prices than others, but I have to believe if you haven't sold it in 2 years there can only be one reason.
I also, as a hobby, buy and sell things online. If I have an item that is not selling and just doesn't seem to be getting any action on it, I have to lower the price. What I have come to understand is that if you are serious about selling something, there are a couple of things you need to decide before you do. Number one, you have to decide that you really want to sell it and are willing to part with it. And number two, you have to price it so people will be interested in it. You say that you really don't need to sell it and are not going to give it away. If you really don't need to sell it and you have it priced at $4000, then you are not going to sell it. No B6100 in good shape and price right will be for sale for two years.
Another thing you can do, if you haven't already, is go see what a couple of dealers might think it is worth. See what trade in price would be as well as what they would put it out on their lot for. Keep in mind a dealers price will be higher than an individual can expect to get for the same tractor since the dealer usually has to do some refurbishing and offer some kind of warranty even if it is just 30 days.
This is just my humble opinion as to why you have not sold your tractor. One mans opinion is usually quite different than every one else's. And usually no one wants to hear them. So take it for what's it worth. And good luck.