Forbes gives him 4 billion at the outside, but they don't get to see tax or other records.
Five or six billion of what Trump claims he's worth is "The Trump Brand". Now, a brand name can have value-- "Apple" is worth a lot. But, just because Trump plasters his name on anything that will let him doesn't mean the name is really worth anything. Certainly not $5 billion.
Some of the stuff he actually owns (which is not a lot) like Mar-a-Lago is worth a lot less than he overpaid for it, but he still increases its value every year for public consumption. What his accountant actually values it at tax time is undoubtedly a lot less and everyone agrees he can't give the place away so it's effectively worth nothing.
For a long time the only property he owned that actually showed a profit was the Grand Hyatt, which made money because he only owned 45% and the majority owner kept Trump's hands off of it. For instance-- he paid $407 million for the Plaza Hotel, dumped another fortune into it and sold it less than 10 years later for $305 million when he divorced Ivana. Some dealmaker.
The last of his bankruptcies ended up with him owning only 5% or so of a lot of his properties. The banks had enough of his bullshit and put a special master, or something, in charge of his finances so that he couldn't do a deal without permission. That stopped him from more assholery like getting conned by NBC into using him to get a better rental deal. He really thought NBC was seriously thinking of moving to the Penn Central rail yards while all they needed was a foil to help keep the rent down where they were. Whatisname in New Jersey who turned the canary food company into a real estate powerhouse, also bid on NBC moving, but he said all along it was only in case the whole thing fell apart and NBC became homeless-- not very likely.