The scale size is 2cm:4m

I don't understand what you are saying

I'm not sure which one is the scale size so i'll solve for both ...

Ratio is 2cm:4m

0.02m:4m or 2cm:400cm also work as ratios

400cm/2cm=200 (so the scale is 200X smaller than the original)

Option 1::

If the scale of the item is:

l=12cm and w=6cm

then we can solve for the actual size by multiplying the values by 200

l=12cm*200=2400cm or 24m

w=6cm*200=1200cm or 12m

Option 2::

If the original item has a length of 12m and a width of 6m then:

l=12m and w=6m

We can find the scale length by dividing by 200

l=12m/200=.06m or 6cm

w=6m/200=0.03m or 3cm