ess91's list "Short & Sweet Names" of 2 great name ideas: Will - Zack!

Short & Sweet Names

Each name on this list is maximum 2 syllables and 4 letters long (the only exception are 1 syllable names longer than 4 letters). 

Will has definitely replaced Bill, not only as a nickname for William but as a stand-alone name as well, partly thanks to England's Prince William, also known as Wills. Currently outside the Top 1000 in the US and the UK, it wouldn't feel totally out of style. 
 Will maintains a nice old-fashioned, down-home charm, and with recent references such as Will Smith, Good Will Hunting, and Will and Grace, it has taken on an air of understated cool.
 Most celebrities known as Will were christened William—Shakespeare, legendary humorist Rogers, cartoonist Eisner, and actor Arnett—but others were not. Will Smith was born Willard and Will Ferrell was originally John William.
 A key character on Glee was Will Schuester.

Will

As the common short form of Zachary or, more unusually, Zachariah, Zack is less intuitive than Zach or Zac in terms of spelling but clearer in terms of pronunciations. Maybe because it rhymes with Jack and Mack, it also feels more complete as a name. Some people also use Zack and brothers as a short form of Isaac.