In you opinion, can I push my AM tuner from its 500-1600 khz to 4000khz
using some of the various mods presented in the previous post?
Or it's really too much?

Tony-

I didn't see your previous posts. Yes, you should be able to modify the tuner,
but it means replacing the front-end tuned circuit as well as the local
oscillator's tuned circuits.

Another approach is to add a converter stage between the 4000khz antenna and
the antenna input connector of the tuner. Such a converter might have a
4000khz preamplifier stage, a converter stage and a local oscillator stage. If
the local oscillator were crystal controlled, you might be able to tune the
frequency on the AM dial, with frequency determined by adding a constant.

73, Fred, K4DII