In Hebrew, two different sequences of four identical letters can occur:

 - וווו. This is a word on its own. It means "and his hook". 
 - ממממ. This sequence occurs in words such as ממממן (from a financier), ממממנו (from his financier) and other similar forms. In addition, it occurs in words such as ממממש (from an implementer), ממממשו (from his implementer) and other similar forms.

Fun fact, in ממממנם (from their financier) and ממממשם (from their implementer), the final letter ם is actually just the letter מ in its 'final form'. So these 6 letter words contain the letter מ five times.