【Impassable】 , 【impenetrable】 ,  【impervious】 ,  【impermeable】  are comparable when they mean not allowing passage through.

【Impassable】 applies chiefly to stretches of land or water which cannot be passed over or crossed because of some insuperable difficulty or obstruction.

【Impenetrable】 applies chiefly to something which is so dense or so thick that not even the thinnest shaft (as of light or air) can find its way through.

Often, however,  【impenetrable】  is preferred to  【impassable】  when implying an exceedingly dense growth that prevents passage <an  【impenetrable】  thicket>  <an  【impenetrable】  forest>

【Impervious】 , which implies impenetrability, applies basically to substances or materials which have been so finished or treated as to make them 【impenetrable】 (as to air, water, or sound waves).

When applied to persons or their minds or hearts,  【impervious】 usually implies complete resistance to anything that would affect them for better or worse.

【Impermeable】 implies impenetrability, whether natural or artificially acquired, by a liquid or a gas and incapacity for becoming soaked or permeated; the term applies chiefly to substances (as some clays) which do not absorb water, to cloths treated so as to be rainproof, or to materials which do not admit the passage of air, light, gas, or water.