Hi L.,
My heart goes out to you! Potty training is never easy, but it sounds like you've got your work cut out for you! My best advice is always to put 'em in underwear, which you've already done...BRAVO. It's always easier for us to keep em in pull ups but that will never work!
I'd say at 4 he knows he has to go but just needs a stronger push. My son is 2 1/2 and potty training right now, he does wear diapers at nap/bed time. I have a 5 yr old daughter that goes to the potty with him and that helps because he looks up to her. Is your other son older?
Since he doesn't like to be reminded by you personally maybe you could try a timer.
-Call it his special potty timer
-Let him help you set it
-Maybe 30-45 min to start
-When it beeps we always stop playing to try going potty!
-If he goes you can show him how the timer gets to be set further (1hour or so)
This way he doesn't feel like you're nagging him but it is just potty time!
I definitely agree with stickers and prizes, best of all your applause and hugs and kisses!
However, I try not to reward kids with candy or food. My own personal opinion, of course do what works for you!
Best of Potty Luck,
M.
P.S. just read some of the other comments and had to say please don't shame your child even though this is slightly defiant. A friend of mine was so fed up with her 4/5 yr old she'd make him sit in it (time out), then take a cold shower! Needless to say, he's 6 now and cries a lot over small things and is quite withdrawn. So sad. Even if he's trying to have a control battle don't let him see it get to you, kids naturally test boundaries. Divert his control issue to having control over the timer, stickers, etc. He's clearly a strong person which is good, just needs guidance on where to aim his strengths!
Hang in there, I've never heard of a 6 or 7 yr old doing this, he'll come around soon!