The full picture:
Both Engineer and Merchant start at 4 yields.
Engineer is boosted at techs Engineering (late Classical), Metal Casting (late Classical), Machinery (late Medieval), Industrialization (late Industrial, +2), Ballistics (late Modern), and Lasers (mid Information, +3).
Merchant is boosted at techs Currency (late Classical), Economics (late Renaissance), and Internet (mid Information, +3).
Tutelary Gods adds +1
to Engineers. Nothing boosts Merchants but not Engineers at the same time.
Merchant was just nerfed in last congress (6-39) removing the boost from Caravansary.
@pineappledan's comment: "If we want Merchants to have higher yields we should either increase the base yield of merchants to 5
or have Currency give them +2
"
This proposal is doing the former, but is a specialist giving base 5
in Ancient on a great building (Market) good to have? If anything we should give more tech boosts to Merchants.
I'd start with +2 at Currency, +1 at Guilds, +2 at Economics, and +2/+3 at Corporations.