This will not be an easy one!
The problem is that there are no, repeat no, direct train services between Seville and Portugal. Also most train services from Seville head in the direction of Madrid.
Also even routing via Madrid, will not help as the only International train passing through Guarda is the Lisbon - Paris train and this bypasses Madrid. Even bus services via Madrid will be longer than via Lisbon.
Given that you only have 4 days the fastest way for you to do it would be by hire car. You could run up from Seville to Badajoz cross the Portuguese border and head North via Portalegre & Castelo Branco. This will take about 6 hours. If taking this option you should you should plan to start at about 04:00 a.m. in the morning to beat the heat in inland Spain - this can be + 40ºC mid-morning in August . It is the fastest and cheapest option.
The only other option is bus/rail.
There is a bus that leaves Seville Plaza das Armas Bus Station every night at 23:59 arriving Lisbon Gare de Oriente at 06:15 the next morning.
There is an intercity train leaving Lisbon Gare de Oriente at 8h39 arriving Guarda at 12h40.
To return on the Monday you would have to catch the 06:39 Sud Express from Guarda (I suspect you will have to book in advance as it is a peak period).
This arrives Lisbon Gare de Oriente at 10:54
There is then a bus that leaves Lisbon Gare de Oriente at 11.30 arriving Seville Plaza das Armas at 19:45.
Obviously if the train is delayed you wuold miss the bus and the next one is only in the evening.
The bus services are operated by www.alsa.es and the train by www.cp.pt
You may find some other useful nuggets on my blog at:
http://barlaventostravelblog.blogspot.com
Hope this helps
