I have a Raspberry PI 3 B+, and when I activate the GPIO 4 as output and checking agains a multimeter I get its 3.3V as expected.
If I connect the GPIO4 to one end of a coil (http://www.electronicoscaldas.com/datasheet/JQC-3F-T73-Series_Liming.pdf) the voltage drops significantly, from 3.3V to 0.99V, and of course, the relay doesn't change it state.
I thought that the coil had a huge resistor and again checking with the multimeter I could verify that it has only 24Ω.
When using the pin 1 (3.3V continuosly) I see that the relay gets activated.
What's wrong with GPIO4? Is there another GPIO which I can replace this with?
DO NOTconnect a GPIO directly to an inductive load like a relay coil or a motor. You will destroy the GPIO and then the Pi.