Deaths happen. This was a dice death all the way around. It happens. EL 4 should not be too tough for 3 4th level characters, so its not like you've done a bad thing. The one caution I would give you is that without a tank in the party, combat against tanks is going to be problimatic. They really need a fighter to stand up front and take that sort of thing, and it would be really worth it to bring a 4th person into the group. Otherwise, you as a DM are going to have to focus heavily on problem solving situataions and keep combat to a minimum (and a EL rating or two lower than normal).
But no matter how good the party is, occassionally they are going to have dice deaths when everything that they've planned just for these emergencies fails them because the dice aren't with them. It's part of the game. You can give out action points if you like, but don't expect that to actually solve the problem because sometimes the dice are just too powerful to overcome. If the game didn't involve any chance of death, it would be a totally different game.
Losing a character is tough, especially if its never happened before or its a character that you've been running for a long time and have become attached to. Losing a character is not fun. I don't think there is a player out there who hasn't felt utterly low and dejected at some point after a character died. You as a player have to be able to pick yourself up, dust yourself off, say to yourself, 'It's only a game' and then start thinking of a new character to play. If you take too much offense at what the dice do to you, you're likely to end up giving up the game completely and spend your life running a juice stand.