Are you able to reinstate a patent if it has been abandoned for 4 years due to failure to pay maintenance fees?
There used to be a 24-month limit, assuming the owners could prove that the non-payment was "unintentional", but also "at any time after the six-month grace period if the delay is shown to the satisfaction of the Director to have been unavoidable." 35 USC § 41(c)(1), as effective in 2007.
Today, the statute reads, in part:
35 USC § 41(c)(1)
The Director may accept the payment of any maintenance fee required by subsection (b) after the 6-month grace period if the delay is shown to the satisfaction of the Director to have been unintentional... If the Director accepts payment of a maintenance fee after the 6-month grace period, the patent shall be considered as not having expired at the end of the grace period.