I am trying to craft MK-3 which I guess I need to craft tier 4 and RE. But what is considered the cheapest Tier 4 gear?? does "archive" count? help

If by "cheapest", you mean which one uses the least quantity of resources to give the greatest increase in crafting xp, then it is probably the tier 4 sub-component that all of the tier 4 non-archive craftable items use. Crafting anything else would require those sub-components anyway, so just banging those out is going to be the fastest and cheapest option.