It depends whether you’re talking about script or vocabulary.
Of those four, Turkish (since the reforms of Kemal Atatürk, the founder of modern Turkey) is the only one that uses the Roman script (with minor modifications), so that would easier. (In the centuries prior to the reforms, since the Muslim Conquest, Turkish was written—with difficulty—in the Arabic script.)
Greek would be next in difficulty, and Persian and Hebrew, with quite different scripts that are written right to left, the most difficult. The advantage of the Persian script is that it is based on the Arabic one, so if you can read it, you will be able to read (albeit not understand) Arabic writing.
Persian, however, is an Indo-European language (e.g. brād(ar) = brother; mād(ar) = mother; shesh = six, etc.—see), so in terms of vocabulary it would be perhaps as easy as Greek.
Turkish and Hebrew belong to different language families (Turkic and Semitic, respectively), so the vocabulary would be more difficult (although modern Hebrew has adopted many European words as supplementary alternatives to native ones, e.g. informatziah, qonteqst, alternativah, which can make life easier.)
Ultimately, your decision is best based on what you want to use the new language for.
(See this and other answers to this question at Quora.com)