As a former concrete contractor I'd say as small as it is,just take up the old concrete as has been suggested. Remove any sod or soft fill and replace with gravel and run a compactor on it. 5"thickness is a LOT stronger than 4" ,and on a slab that small,just use a grooving tool and cut your fresh joints where the major stress will be,usually at the outside edge away from the building.
If you don't want it to settle near your building,drill some holes and use 1/2"rebar to dowel the new slab into the foundation. Tape the ends of the dowels which stick into the fresh concrete with a couple layers of furnace tape. This will let it slip enough to keep it from cracking the new edge.
Freeze/thaw can be combatted by pouring a skirt around the edge 4-6"thick and down 12" to keep water from running in under the edge of new work.
Just a few thoughts you can choose to use or ignore.LOL!
And I never poured a 5 bag,3000 psi mix if the finish is important down thru the years. Order 4000 psi and you MIGHT get 517 lbs. cement per cubic yd., but I doubt it.There is no business I know of where it is so easy to cheat the customer as to what he orders than in the concrete mixing business.
They can add flyash,they can blow it up with excess air entrainment, they can short you on volume, they can send you an OLD load,they can use inferior grade aggregates, and unless you have a concrete testing company there,you'll never be able to prove anything. But by ordering the load to arrive early in the AM,you generally avoid getting an OLD load.
The only way I was ever able to keep a concrete company on the straight and narrow was their knowledge that I WOULD core drill and have the crete tested if they misbehaved.
Oh,I forgot......6X6X10 gauge wire mesh would be fine for your light duty app here,but IF you wanna lil'extra strength,line the perimeter with 1/2"rebar.
Finish the concrete however desired,but cure and seal it(roller works well) with Diamond Clear for a good result don-ohio
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