There's a difference between a "good" movie and and "entertaining" movie. They are both not mutually inclusive OR exclusive either mind you.
With that being said, First Blood (note there is no "Rambo" in the title") is a "GOOD" movie. Storyline is good, original plot and idea. Good everything really. Acting... well... Now for the Rambo series. These are (or maybe rather can be. if you like the genre) "ENTERTAINING" movies. Are they "GOOD"? No. Are they a complete and utter popcorn flick with mass carnage, action and shit blowing up? Yes. Are they GOOD at that. Yes. Are they going to win any Oscars for what they try and succeed in being? No. They are POPCORN flicks and rarely is a popcorn flick "good".
All that being said, Rambo IV will kick ass and I am sure are COMPLETELY entertaining films. They will never get GOOD reviews as reviews are driven off a movie being "GOOD".