Question Can i5-8400 reach 4.0GHz? Mine doesn't

If you install/open HWMonitor, perhaps you'll see evidence of brief forays into the peak turbo occasionally, as designed...; as WIndows uses multiple cores quite frequently, it would likely be brief.

It's possible thatif/when on a Z370/Z390 that supports and has MCE enabled, you might be able to get 3.9-4.0 GHz on all cores if you disable 65 W TDP power budget, and/or tinker with values in Intel's XTU, etc...(If on a factory cooler, you'd probably be lucky to maintain even 3.6 GHz on all cores, from what I've read, as the factory cooler jave proven quite marginal, often leading to throttling under even brief but heavy all-core loads)