I noticed that you were disappointed not to find the word "therefore" in John 4:45 in the NIV or TNIV. I was not able to find "therefore" in this position, in any translation at first, but eventually "therefore" was found in the Darby translation. (I was raised in the Darby Brethren and this translation saved me from KJV onlyism which was a good thing.)
I am going to guess, however, that you were referring to the fact that the Greek word oun was not translated and represented by the English word "therefore" in the NIV. Here is the verse,
- ὅτε οὖν ἦλθεν εἰς τὴν Γαλιλαίαν, ἐδέξαντο αὐτὸν οἱ Γαλιλαῖοι, πάντα ἑωρακότες ὅσα ἐποίησεν ἐν Ἱεροσολύμοις ἐν τῇ ἑορτῇ, καὶ αὐτοὶ γὰρ ἦλθον εἰς τὴν ἑορτήν.
When he arrived in Galilee, the Galileans welcomed him. They had seen all that he had done in Jerusalem at the Passover Feast, for they also had been there. NIV
I certainly, in fact, really,
II. to continue a narrative, so, then
III. in inferences, then, therefore,
In fact, the combination of ὅτ'οὖν can be translated as "since" or "then." It seems that the meaning of "therefore" - that there is an inference involved - is only one possible interpretation among several.
I am disappointed to hear what you have said about the NIV and TNIV, based on the fact that you disagree with how they translate oun; considering that they have used a legitimate approach supported by the lexicons and suitable to the context of this verse.
It has been on my heart for several years now that there is a need for reconciliation between the translators of the TNIV and those who have chosen to publicly criticize this translation.
PS Peter Kirk has just alerted me to the overuse of oun as an all purpose connective in the Gospel of John. I was able to find several references to this in recent commentaries through google books.
PPS Please read the comments here and on Piper's post for more details.