  Reference frame independent quantum key distribution (RFI-QKD) has gained
widespread attention due to the unique advantage for practical application, as
it circumvents the need for active reference frame alignment within the system.
However, in comparison to the standard BB84 protocol, the original 6-state RFI
protocol requires a greater number of quantum states to be operated by Alice
and Bob, which is an aspect that merits optimization. In this work, we propose
a 4-state RFI protocol and illustrate that Alice and Bob each require only four
quantum states to perform channel estimation that remains independent of
reference frame deviation, which can proficiently reduce the system complexity.
Furthermore, through numerical simulations taking the finite-size key effect
into consideration, we show that 4-state RFI protocol can achieve a secure key
rate and transmission distance on par with the original 6-state RFI protocol.
Finally, a experiment over 200 km is inplemented to conducted the feasibility
of our scheme. We believe that our protocol can streamline the implementation
of RFI-QKD and thereby contribute to the practical advancement of RFI-QKD.
