DS2 struggled today with his homework about simplifying fractions...
When I tried to explain hm it appeared that he understand that he needs to find equivalent fraction and these two fraction should have the same value. But he doesn't know about factors, divisibility rule, prime numbers, GCF... And I remember very well my DS1 last year had all these topics in Year 5.

How children supposed to simplify fractions if they are not taught about factors in year 4? Using fraction wall? But there were 12/15 and 6/14 among questions...

Factors and prime numbers arein the year 4 curriculum, so is simplfying fractions and equivalent fractions.

Lots of work on times tables looking for common factors would help.
There are lots of games online and board type games which will reinforce factors. You are right; if these basics are not embedded fractions are going to be unnecessarily tricky.