The question is obviously out of a study book. And not a good one.
After October revolution there were 3 parties at power: bolsheviks, left SRs and maximalists(a branch of anarchists). It was named Soviet Power, not bolshevik's one. And the name was correct in 1917 and half of 1918.
There were not 4 measures, Soviet Power took thousands of measures to keep the power. Its position in 1917-1918 was very weak. And the choice of most useful measures is subjective enough.
Soviet Power made two main laws:
Peace. They stopped to fight non only separately from allies, but even before any agreements with enemy.
Land. They took the well-known and thoroughly made project of SR party and made it a law.
It also started preparations to the elections of Uchreditelnoje (Constitutional) Congress.
I would name as a fourth measure of bolsheviks the temporary three-party system. They could not destroy the Peasants' Soviet in October, as they destroyed the Constitutional Congress later in January. Army was not yet on they side. So, they badly needed left SRs at their side. And in the Workers and Soldiers' Soviets they needed anarchists for the same reason. And without the majority in these both Soviets they would instantly lose. Without support of other parties they had majority in the city Soviet only.
That power was officially supported by a Decree on Establishment of the Workers' and Peasants' Government, but we can't count it as one of the main measures - it only wrote down the real fact.
If you don't like my choice, look at the list of their laws and choose yourself:
I think, Decree on the Nationalization of the Banks and Decree on the State Independence of Finland were very important.
As for non-decree measures, IMHO, the arrest of the previous government, official move of the power to Soviets and very careful politics towards neutral troops were extremely important, too, no less than the first 4 measures.