I had the bright idea of driving to Cardiff from Hammersmith on Sunday. Bad move:
The M4 will be closed in both directions over the weekend of 2 July – 5 July 2021 between Junctions 4b (M25 interchange) and 5 (Langley). The closure will start after 8pm Friday 2 July and the motorway will reopen by 6am on Monday 5 July.
There are closures every weekend for a while going forward see M4 junctions 3-12: smart motorway - Highways England
I note though that if I drive from home I go on the M3 and then take the A329(M) that joins the M4 at junction 10. That is beyond the currently announced closure schedule which ony goes as far as junction 9. Rules out the PG-Hammersmith-Cardiff plan for the foreseeable though.