Where can I find data showing 4-week T-bill rates or U.S. savings accounts' interest rates from 1960s to present, or 1970s to present?
Since each bank is able to set their own rates and terms for savings accounts, this is a difficult question.
4 week Treasury Bills only started trading in July 2001. Source: https://www.treasurydirect.gov/indiv/research/history/histtime/histtime_bills.htm
You might want to look at the Federal Funds Yield which is the overnight rate that banks charge each other for loans held at the Federal Reserve Bank of New York.
Recent data is available here: https://www.newyorkfed.org/data-and-statistics
Historical data can be found on the FRED site back to 1954. https://fred.stlouisfed.org/data/FEDFUNDS.txt