Hi Pedram123, and welcome to Piano World!
I'm surprised no one has responded already to your thread with some kind of helpful info for you. That said, I'm afraid any info I can share with you is pure speculation and conjecture.
What little I know about Baldwin is that they made the Hamilton uprights for many years, which had a good reputation for being well built pianos. I have not seen or heard of too many Hamilton branded grands, but I have heard of a few here and there. As far as the dealer telling you that Baldwin made so many different brand names that it is hard to research, I don't think so; but apparently the Hamilton H396 baby grand is difficult to find info on based on the numbers.
That said, (part II
) I think the dealer you are dealing with is correct; the Hamilton H396 5' grand is apparently one of the very early ones built in China after the bankruptcy and sale of Baldwin in the early 2000s. For reference, here is an ad for another Hamilton H396 grand from Piano Mart online. Hamilton H396 ad from Piano Mart
It looks like the typical small grand piano made in China around this time. The only thing that disturbs and confuses me about the pics in the Piano Mart ad is the lettering embossed in the plate, "Hamilton since 1898 A product of Baldwin USA"; which, if made in China, is certainly not a product of USA.
Since they are asking $6000 for H396 in the Piano Mart ad, you may well be getting a good deal on the Hamilton H396 baby grand you are considering for $4000.
Again, this is just pure speculation and conjecture on my part, but I'm only trying to help (since no one else has responded to your thread yet
). If you like the piano and feel good about the piano, the dealer and the deal, it sounds like all is well. As far as obtaining more info from the serial numbers, chances are a Baldwin dealer would have to help you with that. There are a few members here who are Baldwin dealers. Maybe they will chime in and help.