By Dan Durkin-

Editor’s note: This is the fourth part in a series that takes an all-encompassing look at the state of the Bears’ roster. Click here to read breakdowns of the other positions.

(CBS) After finishing last in the league in sacks (31) and rush defense (2,583 yards) in 2013, the Bears applied to the tried-and-true approach of (re)building from the front to the back this past offseason.

In free agency, $35 million guaranteed was spent on three defensive ends – Lamarr Houston, Willie Young and Jared Allen. In the draft, second- and third-round picks were used on defensive tackles Ego Ferguson and Will Sutton.

Considering the concerted commitment, the Bears didn’t get the full return on their investments they hoped for on the field.

While the run defense improved, their sack production numbers only increased by a half-sack per game. Their rush and coverage units didn’t work in tandem often enough, and they finished as one of the league’s worst pass defenses.

However, there were some notable individual performances. After signing the least expensive free-agent deal, Young emerged in an expanded role. Young’s 10 sacks were both a team- and career-high total. Unfortunately for Young, his strong effort was marred by a torn Achilles in Week 16 against Detroit.

On the interior, the Bears got consistently strong performances from veteran tackles Jeremiah Ratliff and Stephen Paea.

A concussion and knee injury limited Ratliff to 11 games, but when he was on the field, he was clearly the defense’s most impactful performer. Ratliff was primarily used in the one-gap three-technique role, flashing first-step quickness to press the pocket and power to strike, separate and anchor against the run. Ratliff’s 6.5 sacks were the second-most on the team and ranked him sixth among all defensive tackles league-wide.

Paea, who’s set to become an unrestricted free agent, turned in his best performance as a pro. Known primarily as a stout run defender, Paea flashed in pass rush sets. The pair worked effectively as the picker and poster on interior stunts, and Paea effectively used his elite close-area space to knock blockers off their base with his hump move.

Rookies Ferguson and Sutton occasionally displayed some of the power and speed traits that prompted the team to spend early picks on them, but they look to be rotational players.

Looking ahead, Paea is the only unrestricted free agent in the group. From a numbers standpoint, the defensive line appears to be in good shape. However, with a potential scheme change pending to a 3-4, the personnel department and coaching staff must now project how the current group fits the profile of athlete they’re seeking for the new techniques and roles.

In a 3-4 front, the down linemen consist of the four- and five-technique defensive ends and the zero- or shaded-one technique nose tackle, which is the engine that makes the scheme go.

Of the current group, Ratliff and Houston have both played in a 3-4 at the professional level. Ratliff was originally an end with the Dallas Cowboys, but an injury to Jason Ferguson prompted a position switch to the nose, and Ratliff flourished. Houston played both on the edge in Oakland as well as a weak-side rush outside linebacker.

Despite being undersized for a nose tackle, Ratliff’s quickness and ability to convert speed to power was a boon as he became a four-time Pro Bowler and two-time All-Pro nose tackle. With one year left on his deal, Ratliff gives the Bears a potential anchor in the middle to ease the transition, but a younger prospect at the position must be discovered.

Ego Ferguson is the most intriguing young 3-4 prospect on the roster currently. He has the length and strength to play on the edge as a four/five-technique defensive end, but he also flashes the quickness to potentially man the nose as well.

Undrafted rookie Brandon Dunn, who made the most of the limited snaps he got down the stretch, also played nose tackle in college in a 3-4 front.

In the draft, Washington’s Danny Shelton (6-foot-2, 343 pounds) is the top nose tackle prospect and projects to be a top-10 selection.

The Bears have some potential misfit pieces in a 3-4 scheme, like Allen, Young and Sutton.

Allen has been a hand-on-the-ground right defensive end in a 4-3 scheme for the duration of his career. He isn’t stout enough to play defensive end in a 3-4 scheme and hasn’t been used as a coverage dropper, which he’d be asked to do as an outside linebacker.

Considering the financial commitment the previous regime made to Allen, the team may be forced to find a way to utilize him in 2015. New defensive coordinator Vic Fangio has been masterful at putting players in the best position to succeed, so Allen may be utilized in sub rush packages.

Young emerged as the team’s most consistent pass rusher last season. However, he has a long rehabilitation schedule to work through and his skills may be minimized playing defensive end in a 3-4.

Players like Cornelius Washington, who played both defensive end and outside linebacker in a 3-4 front at Georgia, and David Bass could potentially be considered outside linebackers in a new scheme.

As is the case with any defense, it all starts up front. Thus, critical decisions loom for the Bears’ personnel department and new coaching staff as they reconfigure the defense.

Dan Durkin covers the Bears for CBSChicago.com and is a frequent contributor to 670 The Score. Follow him on Twitter at @djdurkin.