Why were the 1st two amazing, but the 2nd two were unwatchable?

Shane Black wrote the 1st one and most of the 2nd one.

That's probably why.

Mother is the name for God on the lips & hearts of all children -Eric D. Raven

Same reason why Alien 3 and Alien Resurrection pale in comparison to Alien & Aliens. The law of diminishing returns.

The first two definitely felt grittier and set standards for cop buddy movies, but I still enjoy the last two. I guess they were lighter in reflection of the changing times.

First two were better because they had better villains and were less goofy. The original is still the best, but part two is almost as good, just more flawed.

Part three had some great moments and the acting was still good, but it was beginning to veer into jokey territory, what with the fat female cop who was attracted to Roger, Leo dying his hair blood and having an ear ring, some of the scenes with Lana, and the villains were quite as good.

Part four never had a complete script during filming, and a lot was made up as they filmed, and a lot of it was improvisation, and not always good improvisation. The highway chase was pretty decent, despite Mel's stunt double being very noticeable at times, and the dock fight between Mel, Danny and Jet Li was excellent, but on a whole the movie was only so so. FAR too jokey, and the Chris Rock addition was annoying and distracting. They tried too hard with that one.

Hopefully Lethal Weapon 5 is a return to the original movie :)

The 2nd one falls into that "bigger is not necessarily better" category for me. Unlike the original film, the sequel went too hard for the laughs and the overdid it with the action sequences and villains. The bad guys in the first movie just seemed like cold-blooded mercenaries, while the 2nd movie went for the racism angle to make them very unlikable. The kills in the first film felt more shocking, while the second one was just kill, kill, kill with no time to breathe.

The first two are awesome and the 2nd two are awesome, just not as awesome as the first two. They're very watchable. Don't know why you think they're not, they are extremely similar and very much like the first two just look more modern due to filming later in film advances.