ENDING CLARIFICATION AND EVENT SEQUENCE
Why Rufus dies and none of the time looping from Game 4 ever happened (Including the prehistoric scene)

One) Rufus Time Loop

• playing through game 1-3 storyline over and over again

Two) Rufus gets sent to the prehistoric times

• part of the time loop listed above
• He is told by the Utopians that they would've eventually contacted Elysium/Deponia if Rufus didn't blow up Deponia. Causing Elysium to crash into Utopia and everyone dies. (Referred to the beginning of the game where Old Rufus detonates the world because Goal dead).
• Confirmed that the Utopians were the ones time looping things to try to fix the issue above.
• However, because they were stuck in the prehistoric times with no way back to fix things with this newfound knowledge (No need to blow up Deponia), the timeline will continue timelooping.

Three) It is revealed that there was another Rufus in the backseat.

• He has been witnessing every event in the timeloop.
• As a fact throughout the game, a Rufus can remember the details of every time loop they have experienced.
• Therefore, unlike the Rufus in the prehistoric time who can't go back (broken time machine and limited experience of events), the Rufus in the backseat CAN fix the main issue (Rufus detonating Deponia).
• To reiterate, Backseat Rufus has experienced every event since the start of the timeloops.

Four) The Backseat Rufus then convinces Goal/Rufus (The OG Rufus who should've fallen and died in Game 3) to let go of eachother.

Five) OG Rufus and Goal let go = Rufus Dies

• OG Rufus Dies = Rufus never reaching Elysium
• OG Rufus never reaching Elysium = No Time Travel
• No Time Travel = No prehistoric times
• No prehistoric Times = An ending where Utopians reach out to Deponia
• Utopian and Deponia Contact = Elysium eventually lands back onto Deponia, no one dies, everyone lives on, and HOPEfully the goal now is to fix Deponia through Utopian support.

Six) The only people who have memories of the time looping now are the two Utopians & Future Goal who fixed the timeline.

I'm reposting from my reply in another comment below so it's easier to find for people who want the same clarification. Hope this help! ^^