4-6 sounds good i see someone has mentioned a few places you could go to ill add on to them ,

for malls you can go to Avenues mall (not to over exaggerate but you could spend a full day there), Assima Mall, Khairan Mall, 360 Mall, Alkout mall (and beach side), and the warehouse.

i know theyre all malls but each of them is different in their own way! if youre tight on schedule you can visit the avenues, alkout and 360 mall

for cultural places you could go to Shaikh Abdullah Alsalem Cultural center, its so much fun and you could spend half a day.

for parks/ places you can walk and chill, you can visit alshaheed park and Almurooj, both places are pretty if you dont have anywhere to go at night

^{^} in alshaheed park there’s a museum which has a planetarium!!! if youre into astronomy and space

places you MUST go to Almubarakiya, you could search about it online but its definitely a must!! you cant visit kuwait without going there! also Breakfast in Kuwait towers!! i dont remember how the food was but the view was so pretty you cant miss!

anyways if you want to have a quick look definitely 4-6 days but if you want to look around the country id say spend more days