In my game I am not able to build lvl 4 houses, instead they are ?? in the build menu. I believe the requirement is to have built all the lvl 3 houses and I have done this, but I do not get the steel factory either.

Is there a solution to this?

Are you playing with that level of housing enabled? I seem to remember that at the start of the game you can choose what the maximum level housing is.

Do you still have a gadget factory, and your wood, cement, and brick factories