The calendar has flipped to October, so it's a good time to take a look back at the first month of football and see what went right and what still needs improving for the four teams in the NFC North through the first four games of the regular season.

Where do the Packers, Bears, Vikings and Lions rank statistically toward the top of the league and the bottom of the league? Let's find out:

GREEN BAY

Record: 3-1

Last week: Green Bay 27, Pittsburgh 17

Top of the rankings:

1. Green Bay's defense has been really good this season limiting the big plays. Through four games, the Packers have allowed just one run of 20-plus yards and seven such passes. Their eight plays of 20-plus yards allowed tied for second in the NFL with Philadelphia behind only Buffalo (6). The NFL average is 15 of those plays given up by defenses across the league.
2. Averaging just five penalties per game, Green Bay trails only the Los Angeles Rams (3.0) and Indianapolis Colts (4.25) for the third lowest average in the NFL.
3. The Seahawks and the Packers are the only two teams that have not recorded a dropped pass on the season.

Needs improvement:

1. Green Bay's defense has to improve on limiting touchdown drives for opponents in and near the red zone. According to STATS, INC., opponents have reached inside the Packers' 30-yard line 14 times, and Green Bay has allowed 11 touchdowns on those drives. That's a touchdown percentage of 78.6, which ranks 31st in NFL.
2. The Packers rank 29th in the NFL in first-down efficiency. They're gaining at least four-plus yards on just 44.5 percent of their first-down plays. That ranks 29th in the league.
3. Big plays of 10-plus yards have been hard to come by for Green Bay's offense. They rank bottom six in the league with 44 such plays.

CHICAGO

Record: 2-2

Last week: Chicago 24, Detroit 14

Top of the rankings:

1. Always known for a terrific pass-rushing defense, Chicago currently leads the NFL with 15 sacks on the year.
2. All those sacks have helped Chicago force 149 negative yards for opponents, the second most in the NFL. Chicago's forced 28 total negative plays in four games.
3. The Bears have three rushes of 20-plus yards on the year, which trails only Baltimore (7), Cleveland (4), Dallas (4) and Buffalo (4).

Needs improvement:

1. The Bears have allowed 47 points outside of the red zone (five touchdowns and four field goals), which are the most in the NFL. They need to better contain some of the big scoring plays.
2. Chicago's 27 passing first downs rank last in the league. For comparison's sake, Tampa Bay leads the NFL with 72.
3. Chicago has converted 13-of-45 third downs for a 28.9 conversion percentage. That's last in the NFL.

MINNESOTA

Record: 1-3

Last week: Cleveland 14, Minnesota 7

Top of the rankings:

1. Minnesota has turned the ball over just twice in four games (one fumble and one interception). Only Seattle has turned the ball over less with one turnover to their credit.
2. The Vikings' offense, with arguably one of the top receiving duos (Adam Thielen & Justin Jefferson) and pair of very good backs (Dalvin Cook & Alexander Mattison), can grind a defense down. They lead the NFL in average plays per scoring drive (9.75).
3. Minnesota's 13 sacks on defense are tied for the fourth most in the league.

Needs improvement:

1. Minnesota's been one of the best blitzing teams in the NFL since Mike Zimmer took over as head coach, but so far this season teams are exploiting the Vikings' blitz. Opponents have completed 15 of 20 passes this season against the blitz for 316 yards with three touchdowns and no interceptions. The opponent passer rating of 156.25 vs. the blitz is the highest in the NFL.
2. The average starting field position for the Vikings has been their 23.1-yard line. That ranks 32nd in the league.
3. The Vikings' offense has converted third-and-short (less than four yards) on just four of their 11 attempts on the year. That's the second lowest percentage (Denver).

DETROIT

Record: 0-4

Last week: Chicago 24, Detroit 14

Top of the rankings:

1. Detroit's defensive front played especially well against Green Bay Week 2 and Baltimore Week 3. On the year, opponents have converted just one third-and-short attempt less than four yards.
2. The Lions have generated 19 explosive plays of 20-plus yards (17 pass, two run), which is tied with Arizona (4-0) for the fourth most in the NFL behind Baltimore (26), Las Vegas (25) and Tampa Bay (24).
3. Lions wide receiver and return man Kalif Raymond ranks top five in the NFL with a 10.0 yard punt-return average. He's one of only six punt return men in the league with an average of 10.0 or higher.

Needs improvement:

1. Detroit's converting on just 33.3 percent of their third-down attempts on the year (15-for-45). That's tied for 27th in the NFL.
2. The Lions are allowing on average 6.63 yards per play to their opponents. Only Kansas City (6.87) is allowing more.
3. Opposing passers have a 116.4 passer rating against the Lions' defense through the first four games. Only Jacksonville (116.8) and Atlanta (120.1) have allowed a higher rating over the first month of the season.