What does "RC" mean in `=SUM(A4:INDIRECT("RC" & G4, 0))`?

You need to read the documentation of the INDIRECT function

INDIRECT(ref_text, [a1])

The INDIRECT function syntax has the following arguments:

• Ref_text: Required. A reference to a cell that contains an A1-style reference, an R1C1-style reference, a name defined as a reference, or a reference to a cell as a text string. If ref_text is not a valid cell reference, INDIRECT returns the #REF! error value.

  • ...

• A1: Optional. A logical value that specifies what type of reference is contained in the cell ref_text.

  • If a1 is TRUE or omitted, ref_text is interpreted as an A1-style reference.
  • If a1 is FALSE, ref_text is interpreted as an R1C1-style reference.

Since your a1 is 0, the reference is now in R1C1-style. In this style RxCy refers to row x column y, so R1C2 is equivalent to B1. If you omit the row number then RCx is the cell in the same row in the x^th column

For more information about the RC-style read

• The beauty of the R1C1 Reference Style
• A1 or R1C1 Notation
• Columns and rows are labeled numerically in Excel
• Excel – R1C1 Reference Style vs. A1
• Reference cells and ranges
• ADDRESS function

In the A1-style RC1 is still valid though. If you scroll to the right then you'll see that after column Z is column AA, AB,... AZ, BA,..., BZ... all the way until the last column (XFD for column 16384). RC1 means the cell in the 470^th column of the first row