If i upgrade to Platinum...

for 1 year, and after 4 months for example i decide to upgrade to Diamond, what is the charge?

I think the system will tell you how much discount you will get. I renewed/Upgraded before the old subscription went sour and I got a reasonable discount.

i c

Based on my ex( upgraded from gold to platinum ):

Initially u pay ~48$ for Platinum/year.

After 4 months, your remaining credit will be 32$ (48$ - 4x4$ )

If you upgrade to Diamond annual( another one year from that date ) which costs ~96$, you'll have to pay $64 ( 96-32 )

Staff can corect me if am wrong.

so there's no discount on the 4 months i had of just platinum?

The idea is that in 4 months from now you will still have 8 months of platinum remaining. The value of those 8 months will be deducted from the value of 1 year diamond when you upgrade.