With your 4 card you are usually going for the jugular, so you want to win the combat and then to do as much damage as you can. Having that in mind:
1. Red Viper - 2 swords is max damage you could possibly deal directly by the 4 card, but just in case you do happen to lose, one fortification comes in handy
2. Eddard Stark - similar to Red Viper, but loses due to not having fortification
3. Stannis Baratheon - no swords sucks, but having potential 5 strength is not bad either. I think Bara spends most time on top of the Iron throne track, so his special ability doesn't get into play often enough to get Stannis to the top of the list though
4. Euron Crow's eye - just one sword is not ideal, but still it has the ability to kill knight, boat or footman unlike
5. Mace Tyrell - kills an enemy footman immediately, if one is present, thus increasing your chance of winning the battle, and as added bonus footman is killed even if you lose, which is nice, but you are not really looking to lose when you are using your 4 card, so this is usually not that great of a benefit. Finally if enemy is attacking with just one footman and manages to win, they can't occupy combat area, which also comes in handy from time to time
6. Tywin Lannister - while it's nice getting 2 power tokens, especially for Lani who doesn't have many areas to cp safely, I prefer other cards skills and bonuses