Most people have sex more than once.

If your risk is 138/10000 per exposure and you have sex with your partner once a week for a year, that's about a 50% chance of infection overall. (Assuming the infection risks are independent, which they might not be.)

If it's 4/10000 per exposure then that's a 2% chance of infection after a year, which for sure isn't huge but I personally would take an annual 2% risk of getting HIV quite seriously.

(Obviously the "once a week" figure is an arbitrary choice for the sake of simplicity, and I am well aware that some people have a lot more sex than that and some have a lot less.)