I dont think the developer is working on DevHook any more. The highest firmware the latest verison of DevHook can only emulate up to 3.03
Custom firmware is safe to use. Ive used numerous version of Dark_Alexs CFW and ive never had any problems with compatibility issues or every messed up the firmware. If the firmware does become corrupt there is an option to launch a recovery menu as the PSP boots up. Allowing you to launch an update to downgrade back to 1.5 or to completly rewrite the psp with a fresh copy of the CFW