SANTA CLARA, Calif. -- San Francisco 49ers defensive end Nick Bosa has been in the NFL for only 169 days, but it didn't take him long to understand that the journey to the top goes through Los Angeles.

This week, all roads point south from the Bay Area to the City of Angels, where the 4-0 Niners are set to meet the 3-2 Los Angeles Rams, the same Rams who are the defending NFC champions and two-time defending kings of the NFC West.

The crown resides in Los Angeles. The 49ers want it.

"This is the first division game," Bosa said. "They've had our number the past couple years and it's time to flip the script."

Calling a Week 6 game a must-win is undoubtedly overstating it, especially when it involves two teams that could be in the mix deep into the season. For the 49ers, this is the first game on the schedule that reasonable minds can't question when it comes to their opponent's legitimacy.

Yes, the 49ers are unblemished through four games for the first time since 1990. But their detractors have found a way to discredit each of their wins.

Beat Tampa Bay in Week 1? The Buccaneers aren't that good and quarterback Jameis Winston gave them the game.

Beat Cincinnati in Week 2? The Bengals aren't any good and have a banged-up offensive line.

Beat Pittsburgh in Week 3? The Steelers don't have Ben Roethlisberger and are adjusting to life without Le'Veon Bell and Antonio Brown.

Even Monday night's 31-3 drubbing of the Cleveland Browns, the team that was supposed to be this year's bottom feeder turned contender, left the doubters calling Cleveland the "same old Browns" rather than crediting the 49ers.

But if the 49ers can find a way to beat the Rams on Sunday, especially coming off a short week and without the services of starting tackles Joe Staley and Mike McGlinchey, fullback Kyle Juszczyk and cornerback Ahkello Witherspoon, it should turn even the most skeptical observers into believers.

"I look forward to playing the Rams every time we get a chance to," coach Kyle Shanahan said. "... We are trying to get to where they've been. I know it's a big challenge for us this week. Everyone is talking about our record, but we are four games into the season. It's only four games, which doesn't mean much. It's our first game in the division and the best way to get to the playoffs is to win your division. That starts on Sunday."

That the divisional opener comes against the Rams and serves as the best litmus test for the 49ers to date seems fitting. For many years, the 49ers and Rams had the premier West Coast rivalry in football. Those days are long gone, though, as the NFL's cyclical nature left the Rams struggling when the 49ers have been contenders and vice versa.

Sunday represents a significant departure from that. This is the first game in which both teams have records above .500 since Week 13 of 2001. How long ago was that? Kurt Warner led the Rams to a 27-14 victory against Jeff Garcia and the 49ers.

ESPN's Football Power Index, which rates the quality of each week's games based on how good each team has been and the game's significance in the standings, rates Sunday's matchup an 80.7, which is the highest for a 49ers-Rams game the past 10 years.

Taking it a step further, Sunday's game should factor prominently into the race for the NFC West crown. As it stands, FPI gives the Niners a 65% chance to win the division. A victory moves that up to 80% and a loss drops it to 53%.

With the Seattle Seahawks sitting at 4-1 and also in the mix, the importance of division wins figures to go up another notch.

"I think this division is tough right now and it's been, in my career, it's always been tough," cornerback Richard Sherman said. "I don't think it gets enough credit because nobody's looking for this division to be tough. Like this year, nobody thought we were going to come out playing as well. They thought, 'Oh, my god, the Rams are going to be really good and Seattle's always going to be in the mix and then we'll see what the 49ers are.' And now, we're sitting at the top of the division, the Rams are still who they are, Seattle's playing well, they're 4-1, so this is looking like one of the best divisions in football."

Sunday's meeting also give the 49ers a chance to reverse a recent trend of Rams' dominance. Tossing out a Week 17 San Francisco win in 2017 that came against mostly Rams' backups, Los Angeles has gone 3-0 with an average margin of victory of 15.7 points, including 29- and 16-point wins in 2018.

For the most part, the 49ers are spending this week trying not to put too much more emphasis on this game than the previous four. Still, the Niners can't deny that in a season that could have more big games ahead, this one could have serious implications in the big picture.

"It would be a good step in the right direction," quarterback Jimmy Garoppolo said. "This game is the most important game for us and we have treated every game like that. So, I think if we keep that up, we will be in a good spot."