Are you sure the lift mechanism will work with a longer cylinder? It has to both fit in the available space when retracted, and also not hit anything when extended and the scissor still work properly.
Someone did the math on that setup, in particular, on the scissor mechanism, so that it uses that particular cylinder, for both the weight being raised and the angle.
You might consider at least checking that the scissor itself can support raising the bed higher, by raising the bed, supporting it somehow, disconnecting the cylinder, and then trying to raise it further, perhaps using a winch or comealong.
Moving the attachment points of the scissor mechanism towards the pivot point would do the job, but doing that will reduce the lifting capacity (by how much is determined by how much it is moved, but that's a math problem), and may also be a structural issue, and at least needs someone who can weld properly to reattach it at it's new location, as there is a lot of force involved lifting the bed (much more than the 4T or so that the load may weigh).
If you're going to play with hydraulics, one thing is guaranteed, you are going to play with formulae and associated numbers.
You won't be hauling 4 cu-yd without sides at least 3' tall in a 5' x 10' dump box.
Figuring out the cylinder volume needed is easy. Figuring out everything else gets complicated really quick.
Less than or equal to 4 yards is not much info.
4 yards of brush/trash is a lot different than 4 yards of sand as an example
Reading between the lines I am guessing brush/trash is your payload, having to drag it out.
So I am going out on a limb and also guessing we are talking 3 or 4 thousand pounds worth of load versus over 16000 pounds of sand for a load.
A picture of what you have for a lift setup now would help with any suggestions.
Also is the current setup power up and gravity down or is it power up and down.
Power down would eliminate a telescopic cylinder as an option.
Power down also reduces the need for a large tank since both sides of the piston have fluid on them.
I will also say a cylinder with a longer stroke or a telescopic cylinder is going to take more fluid to extend, so most likely a larger hydraulic tank would be in order.
Yow, hopefully it's not because the manufacturer was sued out of existence because the trailer was poorly designed/built and some accident happened because of it...
Yes I haul less than 4 yards but I do have 12" of wood sides above the 2' walls. I like to think of things on paper before actually trying changes. Sounds like from the comments buying a new ram is not the solution.