378 lb works out to 3x45 + 1x25 + 1x5 per side plus the bar — call it 375 lb in practice, since gyms rarely stock the odd change. 4 reps sits comfortably inside the 2-to-10 range these formulas were fitted on, so this estimate is about as reliable as the method gets. Your 335 lb set represents about 89% of the estimated max.
How far 378 lb can be trusted
Four reps still sits in the well-behaved part of the range. The seven equations return 365 to 390 lb, and most of that 6.6% gap is them disagreeing about how fast strength falls off with reps rather than any of them being wrong. A set of four is also long enough that fatigue in the last rep is visible: if rep four was a grind, this estimate is honest; if it moved easily, 378 lb is on the low side.
Training loads from a 378 lb max
Programmes are written as percentages of the max, not as absolute weights. From 378 lb, rounded to the nearest 5 lb:
| Percentage | Load | Typical set |
|---|---|---|
| 95% | 360 lb | Heavy single |
| 90% | 340 lb | Doubles and triples |
| 85% | 320 lb | Sets of four to five |
| 80% | 300 lb | Sets of six |
| 75% | 285 lb | Sets of eight |
| 70% | 265 lb | Volume work, ten or more |
378 lb is 3x45 + 1x25 + 1x5 per side plus the bar, 375 lb loaded. At this load the 25 lb the formulas disagree by is bigger than a pair of 10s, which is a good argument for programming off the lower estimate — 365 lb — and letting the bar prove you wrong rather than the other way round. Warm-up jumps also get long here; work up in percentages rather than in fixed plate increments.
The formulas behind this answer, and their limitations
The table above the fold is Epley (1985), Brzycki (1993), Lombardi (1989), O'Conner (1989), Wathan (1994), Lander (1985) and Mayhew (1992), each applied to 335 lb for 4 reps; the headline figure is their median. All seven assume a set taken close to failure with clean technique. At 4 reps you are inside the band they were fitted on, so the largest error here is not the equation but judging how close your set really was to failure — three reps left in the tank reads exactly the same to them. They also under-rate strong, experienced lifters, who hold a higher percentage of max at any rep count than the regression average. The full account — the populations behind each equation, what a bounced rep does to it, and why we stop at ten reps — is set out on the methodology page. Warm up thoroughly and use safety bars or a spotter before going anywhere near 378 lb. This is a training estimate, not medical advice.