The numbers I, II, III and IV refer to the type of prevention based on the table below.

Malaria Risk Type of prevention 
Type I Very limited risk of malaria transmission  Mosquito bite prevention only
Type II Risk of P. vivax malaria or fully chloroquine-sensitive P. falciparum only Mosquito bite prevention plus chloroquine chemoprophylaxis
Type III Risk of malaria transmission and emerging chloroquine resistance Mosquito bite prevention plus chloroquine+proguanil chemoprophylaxis
Type IV High risk of falciparum malaria plus drug resistance, or moderate/low risk falciparum malaria but high drug resistance Mosquito bite prevention plus either mefloquine, doxycycline or atovaquone/proguanil (take one that no resistance is reported for in the specific areas to be visited

Source: WHO International Travel and Health 2005