New York-Merrill Lynch & Co. Inc. said Wednesday that it plans to cut 20% of its investment management unit work force by the end of the year. The group, which manages $525 billion in assets, will cut its number of employees to 3,600 from 4,500. Most of the job cuts have already been made. Merrill cut 350 jobs by partnering with State Street Corp., which now provides services for Merrill Lynch's mutual funds. An additional 150 IT positions have also been eliminated. Merrill Lynch, like most of its rivals, has scrambled to cut costs as brokerage and investment banking revenues fall.