That's an interesting question, KFP. Compare the 2004 roster with the 2018 roster.
2004 Boston Red Sox
Position Starters
C - Varitek - 18 hr, .872 ops, 121 ops+
1b - Millar - 18 hr, .857 ops, 117 ops+
2b - Bellhorn - 17 hr, .817 ops, 107 ops+
3b - Mueller - 12 hr, .811 ops, 106 ops+
SS - Cabrera - 6 hr, .785 ops, 97 ops+ (yes I know Nomar started more games)
LF - Ramirez - 43 hr, 1.009 ops, 152 ops+
CF - Damon - 20 hr, .857 ops, 117 ops+
RF - Nixon - 6 hr, .887 ops, 123 ops+
DH - Ortiz - 41 hr, .983 ops, 145 ops+
Bench
Mirabelli, Reese, Kapler, Mientkiewicz, Roberts
Rotation
SP - Schilling - 3.26 era, 148 era+, 1.06 whip, 8.1 k/9
SP - Pedro - 3.90 era, 124 era +, 1.17 whip, 9.4 k/9
SP - Wakefield - 4.87 era, 99 era+, 1.38 whip, 5.5 k/9
SP - Arroyo - 4.03 era, 120 era+, 1.22 whip, 7.2 k/9
SP - Lowe - 5.42 era, 89 era+, 1.62 whip, 5.2 k/9
Bullpen
RP - Foulke - 2.17 era, 223 era+, 0.94 whip, 8.6 k/9
RP - Timlin - 4.13 era, 117 era+, 1.23 whip, 6.6 k/9
RP - Embree - 4.13 era, 118 era+, 1.15 whip, 6.4 k/9
RP - Leskanic - 3.58 era, 136 era+, 1.45 whip, 7.2 k/9
RP - Myers - 4.20 era, 117 era+, 1.47 whip, 5.4 k/9
RP - Mendoza - 3.52 era, 138 era+, 1.04 whip, 3.8 k/9
2018 Boston Red Sox
Position Starters
C - Vazquez - 3 hr, .540 ops, 46 ops+
1b - Moreland - 15 hr, .758 ops, 102 ops+
2b - Nunez - 10 hr, .677 ops, 81 ops+
3b - Devers - 21 hr, .731 ops, 94 ops+
SS - Bogaerts - 23 hr, .883 ops, 135 ops+
LF - Benintendi - 16 hr, .830 ops, 123 ops+
CF - Bradley - 13 hr, .717 ops, 92 ops+
RF - Betts - 32 hr, 1.078 ops, 186 ops+
DH - Martinez - 43 hr, 1.031 ops, 173 ops+
Bench
Leon, Holt, Pearce, Kinsler
Rotation
SP - Porcello - 4.28 era, 103 era+, 1.18 whip, 8.9 k/9
SP - Price - 3.58 era, 123 era+, 1.14 whip, 9.1 k/9
SP - Sale - 2.11 era, 209 era+, 0.86 whip, 13.5 k/9
SP - Rodriguez - 3.82 era, 116 era+, 1.27 whip, 10.1 k/9
SP - Eovaldi - 3.33 era, 133 era+, 1.28 whip, 8.0 k/9
Bullpen
RP - Kimbrel - 2.74 era, 161 era+, 0.99 whip, 13.9 k/9
RP - Velazquez - 3.18 era, 139 era+, 1.45 whip, 5.6 k/9
RP - Kelly - 4.39 era, 101 era+, 1.36 whip, 9.3 k/9
RP - Barnes - 3.65 era, 121 era+, 1.27 whip, 14.0 k/9
RP - Hembree - 4.20 era, 105 era+, 1.33 whip, 11.4 k/9
RP - Brasier - 1.60 era, 277 era+, 0.77 whip, 7.8 k/9
RP - Workman - 3.27 era, 136 era+, 1.21 whip, 8.1 k/9
Obviously the 2018 team was superior in that it won 108 games (and would have been more had they not taken their foot off the gas in Sept). But it's hard to say the 2018 team had a better roster. Seems pretty close.