In the US when they say something like "Cleared to land, number 4" all it means is you can land, after number 3 lands and clears the runway.
While the other airplane is on the ground rolling to the taxiway, they will clear you to land, I often hear, "Traffic Departing Active, Cleared to Land" or similair.
By doing this, it prevents confusion while on short final as to whether or not you received a landing clearance. This way you have the clearance, and if something doesn't look right you can choose to go missed. Normally when they do this, they will first ask if you have the traffic in sight, once you report "Traffic in sight" they will then clear you to land, since now you can maintain visual seperation.
All in all, it is just like any other clearance, in that it is the PIC's responsibility to know whether you can accept that clearance, and that if you accept it and cannot abide (in this cannot land for some reason) you must notify ATC.