4" blocks will be porous, but still use them. What I would do is belt and braces but you could do some of this yourself.
Get the brickie to build it as you want it, ask him to joint it internaly and flush it up externaly.
Then when he has finished get some SBR, mix it in a bucket with cement and a few handfulls of sand and make up a thick emulsion slurry, paint it one way and then the other, then apply two coats of render. Both with the waterproof additive, then when fully dry paint with the piolite masonry paint, Leyland, or other trade paint.Internally paint it with a water based masonry