T4 vs T5

TL;DR Summary: Because Healing T5 is so expensive, Most T5 players should use T4 for KVK Open Field, Ruins and MGE Kill Event.

The FIRST key to getting good battle reports in open combat is maxing your battle research. The difference between having Final Research at 10 vs 9 is 15% more attack, 15% more Defense, and 9% more Health. If your research is not yet max, ALL of your gems should be going to getting your castle to 25 and then academy to 25. And ALL of your general speedups should be going to research to get it maxed at least for the troop types you use the most. You do not actually have to unlock or use T5 troops, but max tech is very important.

T4 vs T5 cost.

To summarize a message Turquoise sent:

If you put 900K troops into hospital divided equally between Inf, Cav and Archers:

If they are T5 it would cost 192 Mil Food, 192 Mil Wood, 144 Mil Stone and 144 Mil Gold ($700 USD cost in resource reserve packs)

If they are T4 it would cost 72 Mil Food, 72 Mil Wood, 54 Mil Stone and 7.2 Mil Gold ($35 USD cost in resource reserve packs)

Good rule of Thumb for MOST players: Use T4 for KVK Open field, KVK Ruins, MGE Kill Event, etc. Use T5 for KVK Flag Battles and ARK of Osiris.