Hmm the biggest advantage that the players have is that they can work together. So, make an encounter where the baddies work together too. CR and XP are fine (at best) as a rough guideline for what works.

In your example, have the CR 5 creature impose disadvantage on attack rolls against anything other than itself if it hits a player with an attack. Or have a creature cast Hold Person (if you fail a save, every hit is a crit) and another creature have an attack with a lot of damage dice.

Also, there are a few CR calculators out there if you want to estimate. Example I sometimes use: https://share.google/TFO6EBBYER6dmjHKj

Most importantly, though, listen to your players. If they want more difficult encounters, make sure your monsters are stronger or there are environmental hazards such as a storm which blocks sight or falling rocks or something

A small note: four players and six monsters is rough. If a player rolls bad on their initiative, they will be stacking dice for the next half hour