Summer's no longer merely knocking; it's torn the door off the hinges and barged right in. All those weeks you had to build your biceps and chisel your calves are gone, and now, just like that, your weekends are packed and there's no more time to get huge for the beach. The sun's out, but what about your guns? What's a guy who wants to put on size but is suddenly short on time to do?

That's exactly the question we posed to certified strength and conditioning specialist (CSCS) Chris Smith, founder of Train Better Fitness in New York City. His mass-building hack is a four-day program that utilizes a concept called "density training."

This unique plan allows you to get more work done in a shorter amount of time, so you won't need to follow a time-consuming six-days-a-week program to make the rapid gains you're looking for. And while you'll be training hard, you'll be out of the gym in less than an hour each day!

You'll hit your upper and lower body twice a week with high-volume weight workouts, netting you three days to skip the gym entirely and enjoy some summer fun. Don't think you're going to get bored out of your mind repeating the same workouts, either. Each workout in this plan has a slightly different target and emphasis.

One warning before we get into the nitty-gritty: You're going to work freaking hard. We're talking quads trembling, forearms frying, grunting like you're moving a monster truck hard. The reason is that density training has you pushing as hard as possible for short bursts of time. But remember, you'll do it only four times a week, so keep your eyes on that prize: a guilt-free weekend.

Ready to put on serious size and still have a life? Here's how to do it.

Split Decision

To increase your mass, you need to stimulate muscle hypertrophy. Everyone who lifts weights—from your buddy at work to an acquaintance at the gym—has an opinion on the best way to do it. But there are actually many ways. For example, despite what most people think, full-body training can be an effective muscle-building strategy—in fact, it's one that Smith often uses. But to increase the intensity during this four-day plan, he employs splits.

"I program split-style training as part of a volume-accumulation cycle," Smith says. "I use it to crank up the amount of work done for a period of time." Training 1-2 body parts per session lets you home in on those areas and focus on their growth. Because you end up performing more reps per muscle group, you increase your muscles' time under tension, which is an important signal for muscle growth.

During this plan, you'll do two lower-body days—one quad-focused and the other hamstring-intensive—and two upper-body workouts—one for shoulders and arms and one for chest and back. The splits aren't revolutionary—in fact, you've probably done programs with similar groupings before. What makes this program different than other mass-building plans is Smith's approach to sets and reps—the density factor—because it builds in progressive overload.

Timing Is Everything

One of the easiest ways to change your workout—and your workout's volume—is to vary your reps and sets. This program does it with the clock. "You'll perform supersets with a target rep range, but with density training you do as many sets as possible within a time limit," says Smith.

"The goal is to move from exercise to exercise with minimal time in between and try to get in as many sets and total reps as possible," he adds. This forces you to perform maximum work in a set period. Density training will keep you challenged session after session, and it's a useful tool for measuring your progress, too. Your goal is simply to do more work in each time period as you get stronger.

You'll start every workout with a primary movement, done with a traditional hypertrophy-based set-and-rep scheme: 4 sets of 8-10 reps. When you're done, get the stopwatch on your phone ready. For each pair of remaining moves, do the recommended reps of the first exercise followed by the recommended reps of the second; take a break if you need to, and then repeat.

When you're catching your breath between moves in a superset, keep in mind that you're trying to squeeze in as much work as you possibly can. The longer you stand still or scroll through your emails, the fewer sets you'll complete. "Each week, strive to beat the number of total reps from previous workouts," suggests Smith. Continue for the prescribed time (8-10 minutes), and then take the suggested rest.

The Weighting Game

This rule should be set in stone in your head, but it always bears repeating: Choose your weight based on the rep range for each set. Use the heaviest load you possibly can that will let you get every last rep done with perfect form. Bad form doesn't count because it not only invites other muscle groups into the equation, it increases your risk of injury.

If you've got a few more rows to go, and you can barely bend your elbows, simply reduce the weight. Even though the system is based on time, it's not a race: You still want to use great form, the kind that builds muscle. Fast reps, half reps, cheat reps—none of those count.

Because the supersets in this plan are timed, you'll end up doing more volume as you progress and get stronger. Increase the resistance as you get stronger so that you still fall within the target rep range.

Ditch Distractions

Ready to hit the weights? Before you do, grab your headphones, crank up your best get-pumped mix, and prepare to tune out all distractions. You don't have time to swap sweat stories with gym rats or talk up someone at the front desk. When you're working out only a few times a week, you need to get down to business and focus every second on gaining mass. This program will help you get big with just four sessions, but only if you put in the work.

Now do it. Summer's here.

Four Days to Maximum Mass