1. Fill 5 gal bucket 2. Fill 3 gal using 5 gal. This leaves 2 gal in the gal bucket. 3. Empty 3 gal 4. Pour remaining 2 gal from 5 gal bucket into 3 gal bucket. 5. Fill 5 gal bucket 6. Fill up 3 gal bucket using 5 gal bucket. This will leave 4 gal in the 5 gal bucket

Fill both buckets with half of its total volume and combine the content of both :)

From the full 5 gallon bucket, you get the 3 gallons needed to make the smaller bucket full. You now have 2 gallons remaining in the big bucket. Then you empty the 3 gallon bucket and drop the residual 2 gallons from the big one into it. Then you mark the 2 gallon level in the 3 gallon bucket. Now you can drop the two gallons back into the big bucket, then go by the mark to collect two more gallons in the small bucket and drop again into the big bucket, whence you obtain 2+2 = 4 gallons.

Shift some water from the 5 gallon bucket to 3 gallon bucket to make both the buckets have same amount of water. when both the buckets have same amount of water. That means both contain 4 gallons of water.