Car won't start

have the battery TESTED,just because lights,horn,radio,etc are working doesn't mean the battery is good! clean all connections(battery terminals,cable ends,GROUND connections,starter connections,etc) even if they appear clean(corrosion will hide from you)! make all connections look shiny and new!

I would suggest things like loose battery terminals, bad spot in the starter motor, an ignition switch, starter relay but I'm going to assum if the mechanic had it for four days he probably looked everything over. You have a couple options. Drop it off with that mechanic again or get a 2nd opinion. Here is a link to find a shop near you. https://repairpal.com/repair-shops Alex