I don't think that Episode 5 will ever come. Too much time has passed and for me, the interest is lost. Besides that, I think that a moment will come that Vivendi will say "you're not doing anything anymore" and decide to revoke the licence they have given POS regarding the use of King's Quest assets.
The same thing happened also with CBS and Star Trek (original series), although for other reasons but at a certain moment, CBS decided to revoke all the licences for Star Trek hobby projects. Certain good projects were axed because of that and the same thing will happen with TSL here. Shame, but alas . . .