Since external houses and fine nitrogen don't make sense (although I suppose someone might come up with an outlandish situation where that's not the case), and 'high-calorie salad' is probably a contradiction in terms, there is no possible confusion in using
2' high-calorie salad dressing
3' external house wall [though context would usually allow the more
common 'external wall']
4' fine nitrogen droplets
Of course, new cars are common, but here again,
new car tyres [or more usually just new tyres]
would never be read as 'tyres (UK) /tires (US) belonging to new cars'.
The 'rule' in these situations is that a 'noun modifier' (noun used as / as if it were an adjective) is always placed just before the modified noun. (If there is more than one, associations must be examined: eg 'Famous Rugby League assistant manager is fired.'
In some cases, the 'noun modifier + modified noun' will actually appear in a dictionary as a compound noun; it would obviously be silly to stick an adjective between the component parts then:
expensive particle board / expensive particleboard
*particle expensive board
Admittedly, there are occasions where confusion could occur. Judicious use of hyphens removes ambiguities in these cases:
sweet sweet-shop girl [if you must]