Hopefully by "4th runner" people mean 4th runner from the left. You want it in the cylinders that run leanest, which are 1 and 2 (on the right). Putting the EGT probe after the turbo definitely makes it even more worthless than it would normally be right after the head, if you intend to make tuning decisions based on it's measurement. The only compelling reason to put the probe after the turbo is that when the probe eventually fails (and they will) the pieces won't go through the turbo. And I'm sure any manufacturer recomending you do it this way does so for liability reasons. I've had probes go through the turbo and not damage them, probably because it's pretty well melted at that point and relatively soft.
For a tubular manifold, you definitely want to weld a bung on! For a cast manifold, drill and tap. Most probes or their holders are 1/8th NPT. Drill and tap with the motor running, the exhaust manifold pressure will blow most of the chips out the hole. ~2" from the head on runner 1 or 2 will work.
In the end EGT gauges aren't worth all that much on turbo motors. To make an extremely long story short, they are a much better indicator of what you're ignition timing is than what your AFR is. And we can just log timing these days.
A wideband costs 2-3 times what a typical EGT gauge does, but it's infinitely more useful. If you choose to run an EGT gauge there is nothing wrong with that, they can provide some interesting data (try playing with turbine housing and wheel size) and alert you to a big problem, just be careful about using them for tuning based on some arbitrary absolute value.