Letter From Sylvia
Warning This Section Does Contain Spoilers
This letter from Sylvia seems to be the third letter in the series. She tells Jasper that she drove out to the facility and didn't get past the gate guards, but Dr. Cortes was driving by and saw her in her car and stopped to help her. She specifically refers to the doctor as a female. So we know that Dr. Cortes is female and was alive when Sylvia went to visit the facility.
The order of Sylvia's letters are as follows:
#1 - Letter from episode two is the first letter sent to Jasper with message (I've found a way).
#2 - Letter from episode one is the second letter sent to Jasper with message (Hang in there just a little longer).
#3 - Letter from episode four is the third letter sent to Jasper with message (I will come exactly one month from now. Two thirty in the morning. My next letter will tell you where to go).
#4 - Letter from episode three is the fourth letter sent to Jasper with message (Watch by craft rm windw).
To see how to solve the cypher in Sylvia's letter please visit here.