That 1KB cartridge file is not a Guile program, but a Guile program built the cartridge. What's happening here is that Guile is being used to generate GWAT (the Guile dialect of WAT) that is passed to the Hoot assembler to produce a WASM binary. It's not actually compiling a Scheme program to WASM, which as you mention would require the GC extension which WASM-4 doesn't use. Check out https://gitlab.com/spritely/wasm4-wireworld/-/blob/main/miniworld.scm to get an idea of what's going on.