The roman mechanisms work on the ratio of transferring rotation from the chain cog to the cord drums that move the fabric. It’s the relation between the length of the chain pulled and the distance the fabric’s travelled. So, with a 1:1 mechanism, one revolution of the chain cog equals one revolution of the cord drum. A 1:4 mechanism utilises a gearbox which decreases the ratio by 4, meaning that one revolution (360 degrees) of the chain cog would turn the cord drum inside a headrail only by 90 degrees. Simply put, the 1:1 mechanism is fast but stiffer to operate and the 1:4 mechanism is slow but easier to operate.
Knowing that relation will allow you to determine which mechanism is more suitable for your blind with respect to its width, length and weight of the fabric