Equivalent resistance seen between what terminals?

Thanks for that, it's fixed.

Hint : what does that connection do to R1?

When finding Req we can treat the current source as a open circuit thus Req = (R1 + R2||R3)||R4

This seems like the right answer to me! But it does not cohere with the answer below, from user244617. Is that because of the current source, which I added after that answer was given (previously there was only a connecting wire)? In other words, if there is a connecting wire replacing the current source, you cannot treat that part of the circuit as open?

Voltage Sources like batteries and the grid are ideally 0 Ohms ( but then nothing really is lossless except atoms) Current sources like high impedance which demonstrates the contradiction in your edit.

Thank you very much, Tony. I had a suspicion but did not know. I will return the original below circuit and keep the amended one above, which I take has been correctly answered by G36?

If you put a "short" in place of a current source then Req = R2||R3||R4

@G36 Thank you very much! I think the best thing to do is just return the original circuit, so that user244617's answer stands. But the question I asked in the answer below, which matches yours, regarding why that is, remains. If R1 is shorted by the red wire, how is it that Req is not simply R1, since all current would go through R1, with no current going through R2, R3 or R4?

Shorted means very low resistance (Zero Ohms) path. Thus, all the current will flow via a "short" wire. So the current will "bypass" the R1 resistor IR1 = 0A.

@G36 Thank you, once again! To follow on what you mentioned above, how is it that a current source can be treated as an open circuit, while also being like a "high impedance", as another comment mentioned? Is a high impedance the same as an open circuit?

cccube, I edited the schematic to better match the hand drawn schematic here. I will revert it if you're unhappy with it.

@AlfredCentauri Did you look at the edit history before you revert-revert-reverted this to it's Nth iteration? This question is now even more of a mess...

@pipe, yes, I did. Did you look at the link in my comment?

@AlfredCentauri There was a reason for OP to revert his initial change. You have now invalidated the first answer posted, and tacked on your own answer to your own question.

@pipe, there was a reason I came here, edited, and posted an answer. And that's all I have to say about that.

@cccube yes, "high impedance" = high resistivity path = open circuit. "Open" = points A and B are electrically isolated and therefore no current may pass between them. "Shorted" = points A and B are electrically common and therefore no voltage may exist between them.

@AlfredCentauri Thanks again for your answer. I will create another question with the current source, and that way address both difficulties separately, hopefully adding some clarity. Frankly I am still having trouble with the current division question.

cccube, done. I can see you're still having some difficulty with this but you'll get the hang of it.

@AlfredCentauri Thanks!