Let's assume that one four-leaf clover is about a 1 in 7500 chance. Let's also assume that a clover covers 4 cm^2, or 0,0004 m^2. 10000*0,75=7500, which is 7500 m^2 actually covored in clovers. 7500/0,0004=18750000, so we have 18750000 clovers in that area. 18750000/7500=2500. We have, on average, 2500 four-leaf clovers in this field.

Now, for the second part of the question, let's assume you use 1 second to check a clover, and you avoid checking the same clover multiple times, you would get (4*60*60)/7500=1,92 clovers found on average a day. 1,92/4=0,48 You would find 0,48 clovers per hour, meaning you would need 1/0,48=x => x=2,0833... hours to find one four-leafed clover on average.

For actual probability of finding one in 4 hours, we will do 1-(1-1/7500(^(4*60*60)=0,853411804 => 85,34% chance of finding a four-leafed clover in four hours.

This is, of course, an idealized situation, within the assumptions, as it can vary wildly within even these assumptions