Rent increase calculator RPZ

Can anyone assist with a RPZ rental increase calculation. What is permissible ?
Tenants moved in May 18, rent 1800 pcm. No rent increase applied since.
RPZ calculator gives a rent of 1946. But I thought 4% could be added in 2019, 2020 and in 2021. Is it compounded ? Any thoughts ? Is the RPZ calculator correct ?

The increase is now capped to max of 2% so forget about the 4%.
When you put in the address and dates it will show the old increase you were allowed (4%)and the new increase only allowed now.
The Government changed it so those landlords that did not increase yearly got stung again.