Let's say we have 9'x25' monolithic concrete slab of 5" thickness with a 12" deep by 12" wide "haunch" footer around the perimeter. At 2-1/2" inches below the surface of the slab there is the #4 rebar grid set at 24" on-center. Now, running through one of the 25' footers is a 20' length of #4 rebar that is sitting on chairs that rest directly on the soil. It is encased in the footer but is not attached to the rebar grid above. Can that 20' length of #4 concrete-encased rebar serve as the UFER electrode that has the copper wire attached to it?
#4 or 1/2” rebar can be used as the concrete electrode 20’ is the minimum and you will need to stub it up or connect a #4 awg copper wire to it with an approved clamp usually if connecting to the rebar the wire in the concrete is required to be sleeved as it exits but then the connection is not required to be accessible, if connecting to a stub of rebar that attaches to the 20’ piece again #4 awg copper can be used but the connection has to be accessible (I cad weld these so they don’t require access but not many electricians have the blocks to cad weld any more. But yes you can use #4 rebar. Normally the lower pieces connect to the upper ones or there may be 2 parallel bars the wire ties that hold them in place count as connecting. The length of copper if connected in the concrete also counts.