I too replaced the timing belt when I replaced the head gasket, even though the existing one had done less that 5K miles.
I even replaced the balance belt (biddyrascal: you won't have one). Partly due to the possibility of stretching but more because the alignment marks had been rubbed off! ... there wasn't enough to see where these were on the old belt. (What a faff that one is to fit on a 2.0 TS!)
Can we agree to disagree on some parts .. but leave it as:
1) Check the head bolts CAREFULLY for signs of stretching in the threaded area, especially if the engine has been reassembled several times.
2) Clean out the head bolt holes in the block to avoid compressing foreign matter.
3) Take care not to loose or damage the two head dowels when the head is removed.
4) Replaced the cam belt regardless of mileage (and on a 2.0 TS, the balance belt)
5) Make you own mind up on water pumps .. but if over 5 years old, replace.
On the subject of lambdas, the NTK version is notorious for spurious error codes, so make sure these are Bosch! (I read somewhere the NTK are a different ohm rating which is probably the issue!). I replaced mine with a Bosch LS615 universal .. simple to wire in with no soldering required .. and that one has worked for years, where the NTK was throwing errors after a few weeks.
Cylinder 4: Is that the one by the belts (oil down plug hole?) or the thermostat? Check the insulation on the plug connectors .. they do dry out and crack.