Is it possible? Yes. Is it easy? Not quite.
First of all, if there are any more branches left, grab a few, just to have some extra in case something goes wrong.
The first problem to tackle is the moisture. It is possible to cast fresh plant materiel in resin, but the moisture will be trapped inside and rot the branch over time. Put the branch in a container that's just big enough and gently pour simple table salt over it until it's completely covered. You can find numerous tutorials for drying flowers like that. I would probably let it sit for at least 2 weeks to make sure all the moisture is gone.
Next is the mold. Either make sure it's made of a material epoxy doesn't stick to or apply a generous amount of mold release prior to casting. You should also consider that your mold is quite big and the resin itself has a mass that pushes the mold walls outwards. I would build a cardbord or wood scaffold around the mold to keep it in shape.
Next question is: which resin to use? 4 liters of epoxy are going to be expensive, but the expensive stuff may be well worth it. Resins in general have the problem that they come out yellowish and further yellow over time. If you want a crystal clear cast, you need to use crystal clear resin. For castings I prefer resins with a low viscosity (thin texture) and a long curing time, like 24 hours or more. Those give air bubbles time to rise and don't overheat as quickly.
If you have a vacuum or pressure chamber, that can help avoiding air bubbles. A vacuum chamber only has to be big enough to accommodate one cup of freshly mixed resin. A pressure chamber must accommodate the whole cast, since it has to pressurized during the curing time.
When you prepare the actual pour, please consider your own health and safety first. Only work outside or in a very well ventilated room. Always wear gloves that are safe for the resin you use (nitrile for epoxy).
Read the safety data sheet of your chosen resin, it should tell you the maximum thickness you can pour. This information is important because epoxy cures in an exothermic reaction and heats up. If you pour too much at once, the resin heats up to boiling and starts bubbling, which ruins your cast. If your mold is heat-insulating (like surrounded by a wooden box) cut the thickness in half to be on the safe side. It should be easy enough to calculate the volume of resin you can pour to the maximum thickness.
When actually pouring, mix the 2 components as closely as possible to the ratio given by the manufacturer. Keep in mind that some ratios are given in volume and some in weight. Stir the components carefully (to not introduce air bubbles) but thoroughly. If you have a vacuum chamber, put the cup in to release all air bubbles. Pour in a thin stream aiming for a corner of the mold and let the resin spread out naturally. Dislodge big air bubbles with a wooden skewer or similar tool. If you have a pressure chamber, put the mold in to let it cure, otherwise lightly cover the top to avoid dust or dirt and let it cure.
The next layer should ideally be poured after the first layer has become hard, but still tacky. The instructions or safety data sheet of your specific resin should give you specific information.
After everything is finished, you should keep the cast out of direct sunlight. Even crystal clear epoxy will yellow over time, especially when exposed to UV light. I've had a cast turn amber after sitting on a window sill for a month in summer.