I am sorry for all the problems that you and your puppy are going through. It sounds like you care about her very much.
I think that you have an excellent plan by taking her to the cardiologist for an echo. At this point I would hold off from further internet searching and wait for the results. Let me explain the what a grade 4/6 heart murmur means and I can explain why further internet searching may be frustrating.
What we vets listen for when listening to the heart:
We listen for murmurs and arrhythmias. Arrhythmias has to do with the rhythm of the beats. Here I will describe murmurs.
When you listen to a heart and you hear the "Lub Dub" sound, what you are listening to is the closing of the heart valves (little heart doors if you will). The "Lub" is from the systolic contraction and the "Dub" is from the diastolic filling. (systole is the contraction phase and diastole is the filling phase. If it were a gun - firing is systole and reloading is diastole). Most all murmurs occur in systole. So when we hear a murmur, what we here is a "Whoosh Dub, Whoosh Dub" instead of "Lub Dub, Lub Dub". We then think to ourselves, how loud and audible is that "Whoosh"? We scale that on a level of 1 - 6. 6 being the worst and defined as something so prominent, that you can actually get a palpable thrill when you touch the side of the chest. 1 is the most mild and is takes a very quiet room and lots of listening to hear.
Once we hear the murmur, we also then try to find what position we hear that murmur most prominently or what is called the PMI (point of Maximum Intensity). Was it on the left side or the right side. Was it at the Apical (at the apex or tip), basilar (base), pulmonic or tricuspid area etc...
Sometimes we can also describe quality. There are some old terms like "crescendo-decrescendo", "plateau", etc... The terms "musical" "to and fro" can also be heard. Often this is very difficult to accurately appreciate in dogs.
So what does it all mean?
For example we might have a Grade 2/6 left systolic apical murmur. What this means is that the "Whooshing" sound is mild to moderate in intensity and that we can hear it best at the tip of the heart on the left side when the heart contracts. This does not mean that we have a diagnosis other than heart disease. It does mean that in this example, mitral regurgitation is more likely than triscupid regurgitation or patent ductus arteriosus. Why? because we know that mitral regurgitation is more likely to create a murmur that sounds like our example. It can help us narrow the possibilites.
So back to your puppy.
A grade 4/6 murmur means that the murmur is fairly prominent. The fact that it is louder at 12 weeks versus 8 weeks means that it is not likely to be an "innocent" murmur and should be worked up. However for you to do internet searching we also need to know more about the murmur - was it systolic or diastolic? what was the location?
Regardless of this though, the echo will likely give you all the answers that are needed. It will provide far more than auscultation (listening) can do.
Is an echo needed?
Absolutely. Especially with a rottweiler. We need to rule out Sub Aortic Stenosis (SAS) which may have a poor prognosis depending on the severity. There are many possibilities, so we need to know.
Once a diagnosis is made, then a prognosis can be given. Some diagnoses may give her an absolutely normal life. Some may require medication down the road. And unfortunately some may have a poor prognosis. I wish I could give you more information.
Stay strong and know that you have a good plan. Many, many dogs are born with heart murmurs and have normal wonderful lives. Your puppy appears to be in very good hands with you. Please let us know what the cardiologist says.