The streams are grouped into 3 streams and 2 streams. LRichard suggests that 3 refers to Binah, the third Sephira and 2 refers to Heh, the second letter of the Divine Name and corresponds to Binah. This is the best explanation I've heard on this so far.
I have an idea for why Waite says "four streams" in his
Pictorial Key to the Tarot when there are clearly five (unless "four" is a misprint, in which case it all goes out the window). It has to do with a Golden Dawn doctrine on the Hebrew name for Jesus. By adding Shin to the Tetragrammaton you get Yeheshua, and this is the name for Jesus used by the Golden Dawn and Waite. Yod-Heh-Shin-Vau-He. It represents the four elements governed by spirit. It also corresponds to the Pentagram = 5 = He = Shinkinah.
PENTAGRAM
Initiates in the GD reaching the inner order Adeptus Minor 5=6 grade were first exposed to the document Z-1 which is a commentary and analysis of the Neophyte 0=0 ritual. In it there's an interesting comment in the section "The Symbolism of the Temple." It says "Around the Cross are the Symbols of the Four Letters of the Name YHVH--the SHIN of Yeheshuah being only implied and not expressed in the Outer Order." Members of the outer order were not privy to the use of Shin to create Yeheshuah. Since the Waite-Smith tarot was made for public consumption, Waite says "four streams," the fifth stream being implied but not stated outright.
For whatever it's worth, "Jesus" in Hebrew is Yod-Shin-Vau-Ayin-He, Yeshuah. "Yeheshuah" comes from Kabalistic theory and speculation.