Problem you will find is that you think it's a 6ft x 4ft section and hopefully it is.
I had a small leak on my 2006 831RLSS 1ftx2ft. When I tackled the job of fixing the damaged area, it turned out to be about 6ft x 8ft of rotted material around it that appeared ok but, it wasn't when you got into the job.
Not to scare you away by any means but, just want you to know from experience that the damaged area is almost always larger than you think based on what you can see.
Can you walk on your roof? Does it feel like it has soft spots anywhere?
Eternabond is awesome stuff but, not cheap. The dicor lap sealant is around $10 a tube as well and that adds up. If you have to replace the rubber roof material up top it can also add up.
Again, not trying to scare you away or sound negative but, since I went through it, I can tell you from experience it ended up being more than I thought.
I made out well though with my trailer as I got it for such a good price when I sold it, even with the work I put into it, I never lost any money.
Someone once told me that an RV with water damage is equivalent to buying a car with frame damage. Not sure if I believe that completely or not but, some people would and I can certainly see a little truth to it. At $900 if the damage is only in the area you mentioned and that's all it turns out to be, I'd say you are still in pretty good shape.
2012 Keystone Sprinter
Still sticking around for the good people and advice.