Ribbit achieves a 4K footprint, but that just means the size of the Ribbit VM plus the size of the compacted compiled code. The uncompacted code is stored in RAM in the form of linked "ribs" (3 cell objects, or 6/12/24 bytes depending on the word size) and this is not very memory efficient compared to a bytecode representation. So it takes on the order of 64K RAM to run the REPL, which is about 1000 lines of Scheme code. So a rule of thumb is about 64 bytes of RAM per line of Scheme code. You can use that ratio to determine how large of a Scheme program will fit on your specific device.

The Ribbit design is optimized for a setting where the program is communicated say over the Web before it is executed, so it is important to minimize the footprint = transfer time.