I know that the higher the number the better the bike. My dad has a 4200, so I was wondering what kind of improvments are made to make his bike better? And also what does the number 3500 stand for on my bike?
Over the past several years, Trek has been phasing out the __00 line of bikes.
The higher-numbered bikes are going to come with better components.
They've been basically giving non-number names (X-caliber, Marlin) and larger wheels to those bikes from the top down. All that's left at this point is the rim-brake equipped 3500. I suspect it will disappear shortly.
Trek used to have numbered series of bicycles. The first digit indicated which series. 1X00, 2X00, and 5X00 were road bikes. 3X00, 4X00, 6X00, and 8X00 were mountain bikes. 7X00 were hybrids. The second digit usually gave you a general idea of the component trim on the bike. 1200 was spec'd out nicer than 1000, 3700 nicer than 3500, 7700 nicer than 7100, and the rest.
Within the same category of bikes (say, mountain bikes), 4-series bikes were generally nicer than 3-series bikes, with 6 beating both and 8 at the very top.
The extra 00 on the end is just a marketing tactic, not unlike Chevy and Ram using 1500-2500-3500 designations to top Ford's 150-250-350.
Not all of Trek's bikes at the time had purely numerical naming conventions, and not all of the numbered bikes followed this rule. There were (and still are) a handful of bikes with 3-digit numbers for names. The 820 is a bargain bin high-tensile steel recreational mountain bike. The 520 is a chromoly steel tourer. The more recent 720 and 920 are aluminum tourers.
The ones that aren't straight digits are often a name followed by a number. The Marlin line has the models Marlin 5, Marlin 6, and Marlin 7. Marlin 5 is the base model, bearing Tourney components and mechanical disc brakes. The Marlin 6 gets Altus components and Tektro hydraulic brakes. The Marlin 7 gets a mix of Altus and Acera components, hydraulic brakes, and a lockout fork.
So generally speaking, higher numbers still mean better bike, but it's not as straightforward when comparing different lines of bikes.
As for your 3500? While the 3-series is no more, the 3500 still remains to fill the price point gap between the hardly-better-than-department-store-bikes 820 and the entry-level 29er (or 27-fiver, depending on your height) Marlin 5. During the years of the 3-series, the 3500 and 3700 filled the gap between the 820 and the 4-series. Trek just hasn't gotten around to giving it a more inventive name yet.
What 3500 "means" in terms of what bike shops will convey to customers is that it's a good recreational bike for light-duty trails and comfortable cruises around the neighborhood. It's lighter and more capable than the 820, but will have difficulty keeping up with its Marlin and X-Caliber older brothers over rough terrain.