Is it safe to run multiple (four) mains electric cables in a conduit together (heat considerations)
therobyouknow...  If you google search something like " conductor size allowed in what size conduit "  Like I did just now...You will come up with several charts to look at.. like this one.. https://www.constructionmonkey.com/calculations/electrical/tables/wireconduit 

I'm in the United States, (and a Trump supporter so you may or may not like my answer, Joking..).. but for example if your main conductor size is #2 gauge,(and the wire size and insulation type are printed on the wire itself to out dummies like me figure this out)...then reading across the chart you would need 1 1/4" conduit for all 4 wires to be run inside safely. This assumes you know what type (copper or aluminium) wire you have and what degrees' in Celsius the wire will be operating at. Also what type of conduit your using and what type of insulation is on the wires, (see boxes above with drop down menu choices)..  in the UK you should be able to find on line similar helpful charts.. Good Luck.. (BTW.. it's NEVER a bad idea to go up one size in conduit sizing.. just to be extra safe and in case you need to run an extra conductor in the future).. .