If you don't have existing dowels in the floor of the proposed bathroom... and it can support the load.. can you just insert 2 inches deep dowel bars (10mm) and put 4 inches hollow blocks and add the bars (vertical and horizontal) as you go along?
I doubt 2" would be enough to develop any sort of strength if that's what your after. It might be enough to hold the bars in place during forming of the wall, but they would have no structural application.
What loads are you trying to resist? This will determine the required embedment of the bars. 2" is not sufficient to develop bars to transfer moment but perhaps may transfer some minimal amount shear. I think there may be a 6" minimum into supports.
It will be for a comfort room measuring 5 feet by 5 feet and 7.8 feet high. There will be rebar reinforcement vertical and horizontal using 4 inches concrete hollow blocks. Let's say the bars are not connected to the floor slabs. The bathroom may suffer overturning moments? Do you have to fully moment connect the bottom of the restroom to the floor slabs?
Bars embedded 2" into the floor slab are useless to resist overturning moments but may be satisfactory for nominal shear. Why are there overturning moments in a 5' x 5' comfort room? Won't the walls span horizontally?
As pointed out above a 4" masonry wall leaves little room for reinforcement.