Hello Katana,
I would think that a 4mm blade can be useful in a fight, but then again I know very little about it. Your nimcha is definitely not a toy like mine, but I wonder how it would be used. Does one stab or slash with it? And what effect do the incisions on the spine have on cutting? How does it feel in-hand? Is the large hilt helpful or a nuisance? Mine is ridiculously large and useless, but yours may be functional.
Let's say it is the old, genuine ceremonial/wedding nimcha like VANDOO says, and leave it at that, because I really have no idea if you can fight with it or not
I'll keep my eye out for any more info on these daggers, but I think yours is a pretty good example.
Emanuel