What level of resistance effect is equal to full Protection 4 Netherite armor in Minecraft Bedrock?
In Minecraft Bedrock Edition, Protection IV (somehow) works as if it were Protection V in Java Edition. Normally, Protection gives you
(4 × level)% of damage reduction. So, if you have Protection III on any armor, it gives you
4 × 3 = 12% of damage reduction.
However, for some reason, only in Bedrock Edition, Protection IV gives you
(4 × (level + 1))% of damage reduction - that means,
(4 × (4 + 1))% = 20% of damage reduction.
Apparently, because of bug, Bedrock Edition's Protection IV works as if it were Protection V. Normally, you would only get
(4 × level)% of damage reduction, so you would only get
(4 × 4)%(16%) of damage reduction bonus, but in its current state, Protection IV would give you
(4 × 5)%(20%) of damage reduction bonus. However, it is a bug so it may be fixed in future version.
As a sidenote, if you give your armor pieces to have Protection V, it would work as intended (follows
(4 × level)% formula, so
(4 × 5)%.)
So, the Netherite armor set will provides you 20 Armor Points. Each Armor Point gives 4% of damage reduction, so you'll get 4 × 20 = 80% of damage reduction. If you got Protection IV on each armor pieces, this number goes up to 96%.
Base damage reduction(80%) + Protection IV(80%) (
(100% - 80%) * 80% = 16%) = 96%.
AFAIK, in Bedrock edition, there's no difference between Diamond armor set and Netherite armor set except durability and knockback resistance, so you can just let your Diamond armor set to have Protection IV to get same effect.
In summary, you would need 96% of damage reduction to match the effect of full set of Netherite armor with Protection IV.
So, about Resistance level.
Actually, it's impossible to Resistance to have 96% of damage reduction. Since Resistance gives player 20% of damage reduction at each level, so if player gave themselves a Lv5 Resistance, the player will get 100% of damage reduction, which is practically invincible (still, some of damage source can deal damage to the player. See Resistance page from Minecraft fandom for more info.)
Nearest possible is Lv4 Resistance, but that only gives you 80% of damage reduction, which is same as Diamond/Netherite armor set without Protection IV enchant.