How to merge 2 json files using jq (deep merge and concat array)

The problem is, that you want to merge like * would, but treat colliding arrays like + would. You could prepare those arrays such that merging with * will eventually work as desired, e.g. by pre-processing the addition, and storing it in the last affected array.

# Use with option `jq -s` so that the inputs are collected into an array
reduce (
  # Find highest ranked arrays
  [paths(arrays) | select(all(.[1:][]; type == "string"))]
  # Group those that would collide
  | group_by(.[1:])[]
) as $group (.;
  # Store the addition into the last of those arrays
  setpath($group[-1]; [getpath($group[])] | add)
)
# Perform the actual merge (for only two inputs, `first * last` would suffice)
| reduce .[1:][] as $next (first; . * $next)

{
  "key1": {
    "key2": [
      1,
      2,
      3,
      4,
      5
    ]
  },
  "key3": "aaa",
  "key4": "bbb"
}

Demo

Alternatively, use another JSON processor that provides such merging techniques. For example, mikefarah/yq implements a *+ operator that "merges objects, but appends arrays". Note that yq is primarily a YAML processor (so the manual is focused around YAML examples) but it can also handle JSON input/output (if not clear from the filename extensions, use the explicit -pj -oj flags).

yq '. *+ load("b.json")' a.json