Hi everyone,
During the install of UE 4.26 (or 4.25) I have the error IS-BV04 (during the Cleaning step)
Since few day I tried to understand the problem.
I checked my RAM and my Hard Disk …no problem.
I saw in log the problem:
[2020.12.09-17.32.40:171][484]LogBuildPatchServices: Verifying install
[2020.12.09-17.32.40:420][488]LogFileManager: Warning: ReadFile failed: Count=0 Length=21066 Error=Accès refusé. for file D:\SOFT\UE_4.26/Engine/Binaries/ThirdParty/Python3/Win64/Lib/http/cookies.py
[2020.12.09-17.32.40:420][488]LogFileManager: Warning: ReadFile failed: Count=0 Length=18979 Error=Accès refusé. for file D:\SOFT\UE_4.26/Engine/Binaries/ThirdParty/Python3/Win64/Lib/site-packages/pip/_vendor/requests/cookies.py
[2020.12.09-17.32.40:420][488]LogBuildPatchServices: Error: Build verification failed on 2 file(s)
[2020.12.09-17.32.40:420][488]LogBuildPatchServices: Error: EBuildPatchInstallError::BuildVerifyFail BV04
[2020.12.09-17.32.40:420][488]LogBuildPatchServices: Build verification error encountered: BV04: 2
“Accès refusé” = Denied Access, Fr 
I tried again and again to reinstall UE4.26 (I skipped the redownloading during last tests)
I saw the file “cookies.py” was absent in 2 locations.
I decided to install UE4.26 on another laptop(2) and compare the files installed.
On the other laptop(2), there are the files “cookies.py” and the install was a success.
I copied and paste the file in the good locations on my laptop(1)…but during the reinstall, the epic launcher removed the 2 files.
I decided to lock the deletion of them with authorization in Windows 10.
(I got another error xD )
I removed the lock.
I cleaned the associated keys with Unreal Engine in my regedit.
I fixed dll errors in my laptop.
I updated my windows to the last version.
I checked my drivers and they have last versions too.
I disabled my anti-virus.
I disabled my firewall.
I checked my network connection but, it works because I was able to download with the other laptop(2).
I tried with an ethernet link.
I tried the start safe mode of windows.
I used the default Admin account (disabled by default) on Windows.
I tried to install UE4.26 on my SSD and HDD.
I checked all information about this error on the internet. (I saw only problems linked to Fornite and UE but with hardware problems or settings and for me everything seems to be okay.
I…I tried everything I though before ask here.
I send this message to our universe…
“Help me, Obi-Wan Kenobi. You’re my only hope”