Is it safe to run optimze on mutliple tables in parallel in mysql (percona) I have a slave on which no query is coming. Lot of data has been archived and deleted from almost all tables. Size of tables vary from 80Gb to few Mbs. And there are around 130 such tables. 

Mysql (Percona) version is "Ver 14.14 Distrib 5.5.30, for Linux (x86_64) using readline 5.1"

I am running optimize on them, one at a time, but since optimization is a time consuming process, would it be Okay if I run optimize on multiple tables (say 3 to 4) at a time? I have enough disk space for 3 to 4 tables to be copied in the same partition. 

I am assuming optimize on one table simple locks that table alone and has no effect at all on the other tables, hence it would be perfectly safe. But any caveats and former experience is dealing with this is welcomed.
As KJavalik pointed out in the Comments, it it safe, but Disk IO will increase and may choke other operations. So in case the Server is not serving any traffic/queries, you may try, else be careful.

Also keep an eye on the free disk space as optimize command will copy the remaining data of the table being optimized to a new table, and if the slave is in replication, new data will also take up some space.

So in case you are optimizing 2 tables of 50Gb each, for example, and prior experience suggests that post optimization each table will be around 35 Gb, then its good to have at least 90Gb of free space (35 Gb + 35 Gb + 20 Gb of extra free space for other tables to grow in case the database is in replication)