It's not a Piraiba aka Filamentosum. Here is a old pic of my baby Fila for comparison its about 3-4" in the first two pictures and about 4-5" in the second two pictures. Most Juruense should have been showing striping in some form at 4" so that's ruled out. Attached a pic of my 3-4" one from a while back, it had distinct stripes developing at that point. Fila is pics 1,2,6,7,8 in this pic the Fila is about 13" and 1 year old. Juruense is pic 5
The Jau typically referred to as
Paulicea luetkeni, is ruled out also. Pics attached of my baby Jau for reference. Jau is pics 3,4
I'm leaning towards Brachyplatystoma Rousseauxii
http://www.amazon-exotic-import.de/Gallerie/Raubwelse/Seiten/Brachyplatystoma rousseauxii.htm
or Brachyplatystoma Vaillanti
http://www.planetcatfish.com/catelog/species.php?species_id=1178
I'd be tempted to say maybe a goslinea platynema(brachyplatystoma platynema) but it looks too broad in the nose and body for a goslinea, they are generally more slender like tigrinus. If you got one it will be a lot easier to tell when it hits the 6" or so mark which given a good feeding schedule should be about two months. Check out a few of my other pics on planetcatfish for other pics of juveniles of similar species because I've kept the majority of Brachyplatystoma species at some point or another

as well as a few other giant amazon catfish species like Planiceps ~Trent