I'm planning to put laminate type of floating flooring on a floor. The floor has a sub-flooring of 3/4 inch planks and a sheathing of 1/4 inch plywood. I would like to remove the plywood because its thickness will cause a bigger lip at the junction of the room and the hall. Is it ok to install an underlayment of thin foam and the floating flooring directly on the sub-floor planks (assuming they're pretty level)?
The 1/4" plywood is likely luan. It's used to level the floor, often for a vinyl floor. For a hardwood floor, the main reason to have luan is shore up the floor and reduce any pitting or other slope issues. Whether you can remove the luan depends on the quality of your subfloor. If it has any imperfections, I would keep (or replace) the luan.
We got lucky and were (mostly) able to put laminate over 3/4" OSB subfloor. Some places, though, it required additional leveling. As BMitch said, you probably want to keep the luan.
Once you remove whatever floor covering you have now, go over the remaining surface carefully with a long straight edge such as a beam level. The flatter you can get the floor, the easier your laminate install will be.