Q: Is Patent number: 4728812 still in effect. as of 01/29/2021 ?
A:
No, a "4 million series" patent would be from the 1980s and probably expired years ago. Setting aside continuation applications and other theoretical considerations, U.S. Pat. No. 4728812 likely expired in the early to mid 2000's, assuming that its "maintenance fees" were paid (if not, the patent would not even have lasted its full term and would have expired in the early 1990s).
U.S. patents last 20 years from their filing dates (or from the earliest filing date upon which a patent application makes a claim of priority), which generally amounts to about 17 years of patent life after issuance. However, patents filed before June 8, 1995 lasted the longer of 20 years from filing (with priority claims considered) or 17 years from the date of issuance.
Notwithstanding the foregoing rules of thumb, it wouldn't be wise to assume that a patent is expired based only on its number. Before practicing the claims of someone else's patent without a license, it would be prudent to have a patent attorney formally check the USPTO records.
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