Forti OS recommended

Hello @lamichhane 

Unfortunately, TAC doesn't recommend stable firmware version. Every version might have some issues and might have different impact for every customer depending on his network topology. It is updated in the release note under known issues. We would suggest you to go through the release note of the specific version once before you upgrade and analyze if it would have any impact on the services you are using.

You may refer the below links for release notes of 7.4.0 
https://docs.fortinet.com/document/fortigate/7.4.0/fortios-release-notes/236526/known-issues

Thanks
Shaleni

7.2, 7.4 is still too new IMHO

As a best practice please refer to Technical Tip: Recommended Release for FortiOS https://community.fortinet.com/t5/FortiGate/Technical-Tip-Recommended-Release-for-FortiOS/ta-p/227178

It is still recommended to review the release notes (especially known issues section) before making an upgrade.