US4820036 is lapsed and expired. The best place to look this up is on Google Patents:
Patent lifetime in US prior to 1995 was generally 17 years from date of issuance (after 1995 is generally 20 years from the priority date, or the date of the earliest application to which priority is claimed).
Patent US4820036 was issued in 1989 so it would have fallen into the public domain in 2006
Owners of US Patents are required to pay maintenance fees during the term of the patent or the patent lapses before the end of its term. It looks like this patent lapsed before the end of its term.
You can also look up more detail about the file history for this patent on Public PAIR
For more information overall on how to determine remaining patent life for a US Patent see this question: Patent Lifetime and PAIR