All good

I can understand your request, availability discussion is just something we try and steer away from here.
As for your questions -
(i) come across it (particularly in Melbourne)
I have come across it in Melbourne. Around this time last year there was a bit being sold as methamphetamine, like crankinit said.
(iii) personal effects/experiences
4mar has increased serotonergic effects compared to methamphetamine. This adds another aspect to the rush/euphoria - it can be more intense and reminiscent of a mild mdma high combined with the straight meth rush. I find the 4mar rush has more 'depth' than a straight meth high. The rush isn't energetic as such (but then again neither is very pure meth), it's 'munty', and knocks you on your arse. Once the rush has worn off the effects to me are very similar to methamphetamine - clear headed, focused, etc.
Dosages of 4mar are generally one third that of methamphetamine. 30mg is usually more than enough for your average user, for a night.
As has also been mentioned, it lasts a lot longer than methamphetamine. My rule was always that it took 12 hours after my last dose of meth to be able to sleep; on 4mar, I didn't expect to be able to sleep naturally until 24 hours after my last dose.
Agreeing with Crankinit again, when it comes down to it 4mar and meth are extremely similar. Differences are only subtle, and even these differences are usually only apparent with the first few uses in a session. People who say they find the effects very different are more than likely comparing the effects of 4mar to fairly impure meth. 4 mar is nearly identical to
pure meth. 4mar can't be cut in the same ways methamphetamine can, so samples of it are generally much higher purity than your average methamphetamine.
When I was using I used daily, and they were interchangable for me. When you use daily all differences disappear, I assume because serotonin can't be constantly tweaked like dopamine can.
(iv) withdrawal experiences
Just coming down from 4mar felt to me identical to coming down from meth. As for the withdrawals, well as I said, I was using methamphetamine and 4mar interchangably so it's hard to know whether any symptoms were specific to 4mar. I imagine though that a straight 4mar withdrawal would be very similar to a meth withdrawal, which is pretty horrendous. I was a heavy daily user, and my withdrawals were bad. I was in a lot of pain, as well as the usual symptoms of extreme fatigue and a ravenous appetite.
Quitting that addiction was incredibly hard. The psychological addiction 4mar/meth can give you is absolutely insane. The cravings for me were one of the worst feelings I have ever felt, in that they absolutely overtook me, physically as well as mentally. It took 2 months of being off it before they lessened to a mostly bearable level and I started to feel anywhere near 'normal'. It was also incredibly bad for my memory, I don't have a lot of memories from that time. It caused an absolute lack of motivation in me, so that even doing the dishes was a monumental task. It causes anhedonia too, and nothing else seems worth doing except for more 4mar/meth, and hardly anything alse gives you pleasure. I'm still experiencing a bit of that, nearly 11 months off it.
Edit - I also wanted to include this, a marquis reagent test on 4mar. Unlike meth, which goes orange, 4mar doesn't change colour and bubbles.