if you can , try and insert the 4 terminal ceiling rose into the ceiling void above the present fitting and just bring out the cable direct into the light fitting.
This is done by gaining access from above ( loft or lifting floorboards)
The alternative is to use a junction box for the same purpose.
"Cowboys" use chocolate strip connectors wrapped in insulating tape pushed through the cable hole into the space between the joists ....but it is not the correct way to do it.
sparky man, November 2008