Cantonese jyutping Tone 4 question "throat should be vibrating"
The fourth tone is usually considered low-falling, rather than low-flat.

The fourth tone begins just a little lower than where the sixth tone begins. Just start by producing the sixth tone, and then bring the pitch down as low as you can.

In terms of tone numbers (a 1-5 scale, with 1 the lowest), fourth tone is usually transcribed as 21, third tone as 33, and sixth tone as 22.

That's the reason for the advice about hearing your throat vibrate: the pitch you make should be low-frequency. I'd be happy to follow up on this for you if you have any more questions.