The reason you got stuck on Mission 4 is because you didn't get a high enough score on that mission. You need to get at least a bronze rank (190,000 points) in a mission to unlock the following mission. Here's a write-up on the scoring system I did a while back if you want some low-level details about it: https://www.speedrun.com/007nf_gba/thread/d28xt/1#yf9ua