you could fill them both up, and then pour half out of each. you’d then have 4.
3/2+5/2=4

it can be solved in ths way also-
1)fill the 5.Pour it into 3.(2lt will remain in 5 lt bucket)
2)empty 3lt bucketand drop all 2 lt liquid to 3 lt.
3)again fill 5.andpour it to 3 .3 is full now and in 5 there is 4 lt.

very easy

1) Fill the 5. Pour it into the 3.
2) Empty the 3 and pour it(3 liter) with remaining 2 liter from 5 (so now 3 liter bucket contains 2 liter of water).
3) Fill the 5. Pour 1 liter water into 3 liter bucket (where 1 liter is empty). So now 5 liter bucket contains 4 liter water.

there’s several variations to this problem, it’s a classic though.