Build some small rides and an easy, money-making roller coaster (such as the Shuttle Loop Steel Roller Coaster). Don't worry if your first roller coaster doesn't meet the objective. Your goal, at first, should be to get the money rolling in as quickly as possible.
Once you've started making a profit, try making the first of the ten roller coasters as a cheap type, such as a Steel Mini Roller Coaster or a Wooden Crazy Rodent Roller Coaster. To increase excitement, keep building the track near other tracks (over and under itself), and build the track near other rides and pathways. Also, add tunnels and decorate the station. You may need a second lift hill to keep momentum for the whole 4,000 ft. of track - this invites to building a second station as well, since that increases the capacity. After the first test, keep refining your coaster until it meets the objective. Such a long and exciting coaster easily deserves a price between $5 and $6.
With the first roller coaster done, money should be coming in faster, so make a few more flat rides to keep increasing profits and to repay your loan. When you have enough monetary resources, build a second roller coaster that meets the objective.
Continue building the roller coasters until you have all ten finished. It should take less time for each as you make more and more money from each new ride. Remember that you don't have to connect every coaster to your path system, once you have enough money rolling in, you can build coasters in the back of the park, test, and open them.
TIP: The Steel Roller Coaster can also run more than two trains in a launch configuration. Keep the coaster on the default setting and after you've built the station, just use the booster element. Set the speed to max and create a long enough strip to get the train up to a good speed. To make the objective length in just one boost, you might want to get the strip to launch the train past 50 mph, it would have to be fairly long to get it that fast but it still counts as the length... You can choose to do it in one launch or in multiple launches. Multiple launches are where you have a small strip the shoots the train to let's say 40 mph, followed by a nice set of twists and hills until the train burns off the speed then followed by another launch into another set of twists to finish up the ride. Like this or this and even this. It is rather easy to make an exciting ride using this style of build as long as you don't go crazy with hills and have too much G-Forces, but this style does have a drawback. It is expensive to maintain--$1,100 per hour minimum if you're lucky. However, if you build it right, you should still be able to profit nicely off of it, around $4,000 to $9,000 an hour with $4 a ride: so long as you can move at least 1,200 guests an hour, you're golden with this style of ride.