The FR-AV4 has released the latest version 1.01 for its firmware. Please refer to Release Notes for further information.

You can update the unit’s firmware using TASCAM Update Manager.
TASCAM Update Manager is a free application that allows users to easily update the firmware of TASCAM products.
It supports multiple TASCAM devices and enables centralized management of firmware updates.
TASCAM Update Manager is compatible with DR-10L Pro, FR-AV2, FR-AV4 and Portacapture X6

FR-AV4 Download
TASCAM Update Manager Product Page