How to prepare executon plan for given sql query on a server with 4 processors (Oracle database)
Hints for join operations are used the same way as other hints, for instance

SELECT /*+ parallel(4) USE_MERGE(T1 T2)*/ T1.A
FROM .... 


Optimizer chooses join algorithm based on different criteria. There are many things to be considered, but in general merge join is used when optimizer mode is ALL_ROWS (it prefers nested loops for FIRST_ROWS) , both tables involved in the operation have index on join column[s] ( it may still use merge join even if you don't have indexes, but hash_join_enabled set to false or using hash join estimated as more expensive). 

The query with hint parallel(4) may potentially allocate 8 parallel servers (4 consumers, 4 producers).