BJT circuit question
"I mean I would think the current going through R1 would split between R2 and R4 to some extent."

In that case, good question! The fact that you're asking this shows that you're actually thinking about it.

The answer is that yes, the current does get split, but it's more complicated than that, because the current into the base is not in general the same as the current out of the emitter. In forward active mode, the resistor is effectively magnified by 1 + β from the point of view of the base node. This is because the current through the resistor is the emitter current, Ie, not the base current Ib. Since Ie = (1 + β)Ib, what is 'seen' at the base node is this:







simulate this circuit &ndash; Schematic created using CircuitLab

Now consider what happens if you take a large resistor and put it in parallel with a small one. The resistance of the pair is going to be much closer to that of the smaller resistor--so this "magnified" resistor will have much less of an effect on the output than you might initially think.

Note that this is a fairly simplified explanation and may contain some errors besides--I'm a bit rusty on the details.