How preferable is it to add a third or fourth language I know to my CV? And is it necessary to mention my level in that language?

language plays important role in your resume and employer consider seriously where applicable. level shall be mentioned in order to convey true picture to others (Employers). Hiring  Manager may know languages as described in Resume will definitely put good impact.

As a persion majoring in Translation & Applied lInguistics,it is preferable to mention the languages i do speak or at least the languages i'm learning at the moment.Being a bi\\multilingual person does not only help in having a better job but also indicates that you are a "non-stop self developing person".Worst thing an employer wants is an employee who does not improve his\\her skills & not able to keep up with the rapid changes in the business world.Moreover, a lot of International comapanies have branches in Kuwait whether they are Japanese,German or any other countries.Some of these companies,like the Japanese and most of Asian ones,do not use English Language as a way of communication.Such countries are so proud of their language & culture and here comes the need of having a multilingual employee in order to solve this dilemma and communicate properly without disrespecting the culture or the language of these countries.Not to mention that language and culture are not separable and not every translator is a good communicator until he\\she is clearly aware of the culture he\\she is dealing with.

Mentioning your proficiency in any language is nessesary whereas some companies require fluent speakers whether it is English or any other language.Also,its a preferable way to save your &your interviewer's time who may not find you eligible for the vacant job position you're applying for.

It's better to put all the languages you know with the level 

It is very preferable especially if you are applying for an international job that requires more than one language. 

It is very preferable and if you know International languages then it is necessary to mention the level. This definitely helps in Hiring and knowing a language is a great skill.

It is a valuable skill and highly preferable to add another language. Yes, it is necessairy to disclose my proficiency. 

I think it will be added advantage and sure necessary to mention profeciency skill as well. Thanks.

because i would inderstand more people and communicate with  them and that makes life in other communities easy

YES its very valuble to add another language

as long you speak it why not . i think you should  add every language you speak.all the skills

It is very preferable to add all the languages you have especially when applying to an international company that requires a certain language