I was delighted by the response to my first post regarding Combo Attacks in 4e; Big Ball of No Fun (http://bigballofnofun.blogspot.com/2011/01/combo-attacks.html ) did a great post outlying one of the ways that he would implement this style of attack in his game. I really like what he managed to homebrew up, but I would like to present the rules below as a different perspective based on a comment by Sully, which uses an already built in mechanic in 4e, the Action point.
The first thing that I present is a new category of Feats, the Combo Feat. Combo Feats are unique in several ways. Each Combo Feat has a specific class as a prerequisite. The benefit that they present is that whenever you use a specific at-will power against an enemy, then follow up by spending an action point,to make a follow up attack with another specific at-will attack power, you are given the option of spending ONE ADDITIONAL action point in order to use a specific encounter power given in the Feat. These Finisher encounter powers are slightly more powerful than another power of the same level, but that power is offset by the fact that you have to ultimately burn 2 Action Points in order to use one. This presents an opportunity for players to bust out their big combo on the boss monster and seriously wreck him up, but prevents the player from doing too much damage by spending 2 Action points, in addition to his or her normal standard action, in order to drop 2 dailies and an Encounter power on the boss all at the same time (potentially burning him down in a single round if all the players bust their combos on round one).
Combo Feats are also divided into three tiers, which provide increasingly better Combo attacks and let you spend increasingly more Action points to execute the maneuver. Heroic Tier works as presented above, allowing you to spend 2 Action points per round. Paragon Tier ultimately presents the ability to spend 3 Action points, and Epic Tier allows the use of 4 Action points. This feat breaks two specific mechanical rules: 1) It breaks the rule for using only 1 Action point in an encounter (or round, I can’t remember) and 2) It will also allow, from time to time, to break the rule of using the same at-will attack power in the same turn. Some Combo Feats will allow you to make the same at-will attack power twice before you make your Finishing Attack.
The final aspect of the Combo Feat is the encounter power included in the Feat itself. The Power has several things that make it important: 1) You can only use it after hitting with at least 2 Previous attacks 2) It requires an Action Point to use 3) It is slightly more powerful than an attack of equivalent level and 4) It has a special critical component. Whenever you roll a critical hit with a Finishing Move, the encounter power that you use it with gains an additional boost or bonus.
As an example one potential Combo feat could work like this:
Storming Shield Fury [Combo]
Prerequisites: Level 1, Fighter
Effect: Whenever you hit with your Tide of Iron At-will Power, you may spend an action point to make this attack again. If you hit with the second use of Tide of Iron, you may immediately spend an additional action point to make the attack below.
You send your opponent flying with a massive blow to the torso.
Target: 1 Opponent hit by your previous Tide of Iron Attack
Hit: 1[W]+Strength Modifier damage; push the target a number of turns equal to your strength modifier; the target is then knocked prone.
Critical: Double the damage granted by your strength modifier above and the distance pushed.