I have not seen the newest movie yet but I have seen the other movies leading up to it (and the trailer for the latest movie).
What I would say is that it's optional, depending on what you are interested in.
Let's take a tangent and discuss <PERSON>'s other career-defining role, <PERSON> who's had several movies
* Rocky
* Rocky II (2 Rocky 2 Furious)
* Rocky III (The Fate of the T)
* Rocky IV (Rocky 2 Russia)
* Rocky V (Back 2 Rocky)
* <PERSON> (Rocky Drift)
* <PERSON> (<PERSON> & Apollo)
* Creed II (<PERSON> & <PERSON> Ride Again)
Each movie added to the character arc of the protagonist, each chapter there were new losses, new lessons and in the end, the protagonist came to terms with his lot in life and earned some rest (until the next sequel came along anyway). Regardless, each film was enjoyable on its own as a self-contained unit, started with a small recap of the last film if it was important to the continuity.
Rambo films are slightly different in that the continuity of one films isn't as closely tied to the others but each film reflects how the character is affected by the politics at that time in American history.
Given that you have seen the first two, I'll skip that and will actively not spoil any plot points of the other two movies.
I will say that Rambo III was dedicated to the brave people of Afghanistan (word for word from the closing card of the film) as they fought the Soviet forces in the area.
Rambo (the fourth film) finds a battle weary warrior in the post 9/11 era. Completely detached from American military, finding himself involved in a conflict where he initially has no stakes but is compelled to act since not participating is itself taking a stance (and you can therefore be complicit in the consequences of the actions of others you didn't act to prevent)
From the trailer, there are allusions to his past catching up to him. His new antagonists may be part of drug cartels (which may link it to Middle East Opium/Heroin and his time in Afghanistan, who knows). 569