For this you will need some apps and files all the files and app donwload links is given bleow so you can easily download.
So now let's start.
1. Wallpapaers :
All the Wallpapaers of one ui 4 is given below so download all the wallpapers now set the wallpapers into the Homescreen.
2. Homescreen :
Firstly open the Nova Launcher app so allow all the permissions and then you will come back to the default homescreen so just again open the nova Launcher app. Now tap and hold on hoomescreen and go to the nova launcher settings not set the nova launcher as a default launcher. Now again open nova launhcer app settings and go to Backup and Restore > Restore Backup > Browse then select the One UI nova Backup file then tap on ok.
Now tap to the KWGT widget box that showing in homescreen then you will redirected to kwgt app now tap on import and select the one ui 4 widget files one by one now go to search option then type One ui then you will got all the one ui widgets are imported so select any one widget then save the widget. Now go back to homescreen.
Now the Homescreen completely look's like One UI 4 so move to the next steps ( Note : For this you will need the KWGT Pro version app which is paid for free app link you can message me on instagram @mrnikhiltech ).
3. Notification Panel :
Now open the One Shade app and allow all the permissions now go to the Color section now go to Background Type option then set Transperay : 50 and Blur Amount : 60 now tap to ok. Now go to the Background and increase the Transperacy do same with notification background. Here go to active tile color and set the color according to wallpaper color and go back here go to layout section now go to tile size and set the value to 53 then go back to homescreen.
4. Volume Panels :
For this open Volume Style app and go tap on start now allow the permission here you will see many more volume panel styles so just select the One UI 3 volume panel style then go to the style settings and select the accest color according to the wallpaper color.
5. Caller Screen :
If we talk about caller screen today's almost all mobile brands using the google dailer so you can also use to get caller screen like one ui 4.
Now all the steps are completed and your mobile look's like running on One UI 4 :)
Download Linksk :
Nova LauncherOne UI Icons
Backup File
KWGT Pro
Wallpapers
One UI Widget files
One Shade
Volume Style