can i use unreal engine 5.1 assets packs in 5.4
TLDR: Don’t do it if it’s a lot of money are you’re not sure, but if you already have the asset, just install it and find out 
Yes. Even if it doesn’t have that engine version on the marketplace page.
BUT: Not if it’s a plugin, of course. And also you might have some material compatibility problems, if you push it a long way ( by version number ).
But generally, things like meshes and so on, will be fine.
In the worse case scenario you could install 5.1 alongside 5.4. Open the assets in 5.1 export the models to FBX files and then import them into 5.4. But transferring bp game logic doesn’t usually go so smoothly.
You can sometimes get away with a copy + paste between engine versions but if the asset is more complex then it will be tougher to recreate.
And if c++ is involved then you would need a recompile to a higher version of the plugin. (or do it yourself if source is available)
But if the plugin is only the compiled watered down version then you are stuck at the mercy of the plugin creator to update it. (You can sometimes check if they have their own repo on github where the source might be accessible)