Never heard the expression “fourth kind” but one, two, and three I believe are used often.

One is, I believe, observing the spaceship. Seeing a UFO for example.

Two is having more hard evidence such as finding irradiated materials. Some physical effect the UFO may have had on the world.

Three is observing some beings along with the UFO sighting.

Ah! Fourth kind is an abduction by the beings.

1st kind – sighting of a ufo far off/on radar
2nd kind – close up sighting of a ufo and/or physical evidence
3rd kind- sighting of aliens
4th kind – abductions
5th kind – humans contact ufo/aliens by any means

I can’t tell you about the fourth kind. I would have to kill you.
The first kind is in-law encounters and the second kind involves a rare form of cheese. I can say no more.

I guess this would be a good source: Close Encounter

That’s classified, buster.

watch “Fourth Kind” movie trailer they explain you what this means

5th Kind-Anal Probes
4th Kind-dead cattle in a field
3rd Kind-pot induced high while star spinning. (I hear it’s lame)
2nd Kind-clouds that look like UFOs.
1st Kind-Houston we have a blip!

I thought there were only two kinds – imagined and invented.

An encounter with the Borg might count as the sixth kind.

or perhaps a long, thin probe covered with vaselline.