I would have to say your test kit is bad.
I'm not sure which salt you are referring to, but typically Red Sea salt mixed to about 8 dKH and Coralife salt mixes to about 9 dKH. There is no salt made that mixes to such a low alk reading of 4.2 dKH.
The 4.2 on the bucket is meq/l. Did you actually test a fresh mix? What brand test kit are you using?
Alkalinity can be measured in 3 different readings depending on the brand test kit you are using.
dKH - degrees of karbonate hardness
meq/l - milliequivalents per liter
ppm - parts per million
So doing some math.....
your current alk is 16 dKH (which I find hard to believe and still think it's a bad test kit)
you did a 5% water change (10 ltr of 200 ltr total) using water with an alk level of 12 dKH
you dropped your alk from 16 dKH to 15.8 dKH
Taking into account the accuracy of hobby grade test kits, that small change most likely won't even be noticable.
its a new red sea test kit and ive had it tested by the aquarium shop here and got the same reading with red sea and api kits...and i used tap water as i dont have a source of RO water im in australia