might be BUG NUTS, if the car is meant to be a VW Beetle ("Bug").
surely SCOT FREE (the flag shown is that of Scotland).
I have no idea about at present.
TOP DRAWER, I assume.
well, the busts are of Julius Caesar. It should probably be obvious to me what to do with ten Julius Caesar busts, but right now it isn't. They're laid out like a phone keypad but if that's more than coincidence I don't yet see what to do with it. (CAESAR PHONE -> SOUSAPHONE? JULIUS KEYPAD? HEADPHONE? All nonsense.)
So we have, maybe,
where I'm guessing that
all rebus solutions will turn out to be two-word phrases. The fact that the last letter of the first one is the first letter of the next suggests an obvious way to get four letters, but unfortunately that seems to lead to SETR which, um, doesn't sound like the name of a river in Spain. Another possibility is that the last rebus means "Caesar shift by 10" and should be applied to the results of the other four, after reducing each somehow to a single letter.
I also remark (though I don't know whether it helps) that
the rebus solutions we have so far all look kinda cryptic-clue-y. BUG NUTS could indicate that we should take an anagram of either BUG or NUTS. SCOT FREE could indicate an anagram of SCOT. TOP DRAWER could mean the letter D.
What rivers are there in Spain?
Wikipedia yields the following four-letter ones; I may well have missed some. Ason, Avia, Deva, Ebro, Esla, Foix, Gaia, Muga, Oria, Saja, Tajo, Viar.