Associative arrays: transitioning to Bash 4.0 and maintaining compatibility with older shells

Goal

Create a single Bash script that leverages associative arrays for efficient key‑value lookups while remaining functional on systems that only provide Bash 3.x or earlier, where the -A option to declare is unavailable.

The constraint is that attempting to declare an associative array on Bash 3.x produces a parsing error, preventing the script from starting, and there is no built‑in fallback mechanism within the language itself.

Given this version boundary, what approaches exist to detect associative array support at runtime, how can equivalent storage be emulated on older Bash releases without invoking external utilities, and what patterns allow the same source file to be sourced or executed successfully across both Bash 3.x and 4+?