I think the F4 is a fantastic recorder, and personally I don't think the MixPre-6 is worth the extra money (that's just my personal opinion...don't kill me guys, I'm not saying the F4 is objectively better). The F4 is a tank, and having two SD card slots is great insurance. The display isn't pretty, but I found it to work just fine. Sure, colors would be nice, but I didn't find it to make a functional difference.
Some people might say the MixPre-6 sounds better than the F4, and I'm not going to touch that. For myself, I don't think I could hear the difference between an F4 and a MixPre-6 (I couldn't when comparing the F8 with the MixPre-6, and the F4 likely has the same preamps as the F8). At the end of the day only you can decide whether you hear a difference between the two. Certainly on paper the specs are different, but I just haven't found that to be audible to me.
As for the PMD706, I've followed the thread a bit and it just doesn't seem like a high quality unit. Even with the mods, there's nothing that would make me want to get a PMD706 over other options. Again, just my opinion.
You might want to check out the F8 vs. MixPre-6 comp thread if you haven't already. See if you can hear a difference doing a real ABX comparison, before reading any responses in the thread. Let your own ears make the decision. At the end of the day, you really can't go wrong with either the F4 or MixPre-6, in my opinion.