About every 20 minutes or so, more water is released into the bowl from the tank. Toilet is less than 1 yr old ,model 4021

How old? Who installed? Any warranty? I'd follow that route first.
If not warranty I'd try to get the installer to be interested. Perhaps he will have a "red face" an help out.

check to see if waster continues to run into the bowl after the plunger has returned to stop water flowing. If it is a slow leak then replace the flush washer on the stopper plunger.

Leave the fill line in position that works otherwise the siphoning will continue in the other position.

Replace or adjust the flapper.

Hi keithn60...

I had the same problem one time with my toilet tank, the answer is to put some silicone on the seal washer between the take and bowl assembly and to use a level on the tank to tighten it just enough to be level and not leak.
Please take time to rate me

if you can see the water running in the bowl it's a good chance that the flappers warn http://www.aplumbers.com/residential/toilet
1. tank the tank lid off.2. flush the rubber thing that comes up and allows the water to go to the bowl is the flapper 3. remove 4. replace that should fix it.
problem 2
1. tank tank lid off.2. if the waters flowing over the long tube in the center and down then the water level is too high3. adjust the level either it will have a ball **** or a float in either case   a ball **** adjusts by a screw on the fill valve. the float has a either a metal rod w/ a pinch clamp or a plastic long screw type thing.4. adjust water level .5. barely lift handle to allow a little water out til fill valve kicks on then release handle til valve stops .6. just keep re adjusting til you get the water level right there should be a molded in line for the level on the back of the tank.

There is no easy answer for your problem other than the flapper is not holding the water in the tank It is the only thing that keeps the water from going into the bowl when a toilet is not in use...on ANY toilet. Not all flappers are of the same quality, so be sure to replace it with the EXACT style as came with the toilet.

With the toilet tank topped off, turn off the water supply. Then put a generous amount of food coloring in the tank. Wait ten minutes. If you see colored water in the bowl it will confirm that the flapper is leaking and needs some adjustment or to be replaced.

Hi
try to increase the amount of water for each flush .

Its usually better to buy the set, but if you don't want too that's fine. As long as the water capacity of your old tank is what the new bowl requires. If you look at the set at the store, it will usually tell you the water capacity and specs of the new bowl. Although most bowls use the same holes and bolts to combine the tank and bowl, make sure the old tank will fit on the new bowl you selected.