2. He might (may) come tomorrow.
This suggests that the choice of whether he comes or not is wholly up to him (or that the speaker is ignoring any factors which might prevent him, and just making a flat statement - he might or he might not.)
1. He might (may) be able to come tomorrow.
This suggests that there may be circumstances/other commitments which could prevent him coming (even if he dearly wanted to.)
4. Applicants for the job must speak two foreign languages.
Whilst 3 and 4 are not greatly different, 4 is preferable to 3: it is not that they MUST speak two languages (when? everyday, even if it is "talking to themselves" if there isn't another Norwegian or German speaker to talk to?)
3. Applicants for the job must be able to speak two foreign languages.
Here, it is more that the applicant has the capability of conversing in another language, so that if I walk into your office and don't speak Russian, they will page you and ask you to deal with my enquiries because 'you are able to speak two languages' - Russian and English.
Student or Learner