You might actually have an easier time using 4e as a model for your game, assuming you can come up with a way to generate a simple leveling schema since, honestly, using the standard experience system would be a massive hassle. you would need to simplify it to make it playable by people without having to stop to do math every time something die (I'd probably do it as flat values: 50 additional xp required for each level, so that it's 50 for level 2 and 100 for level 3, etc; NPCs grant a flat 1 experience and Players grant 5+levelx2 experience, so something similar). 4e already has the fundamental set up for your 4 roles: Strikers are your assassins (pick one target and blow them the hell up), Leaders are your support (it's actually explicitly what they do; they have explicit support abilities but even their at will attacks are designed to support as they deal damage), and Defenders are tanks (their tank auras lock down enemies they get adjacent to). There isn't an explicit carry since the roles/classes are designed to be balanced, but you could create class constructs that have better scaling as they level to account for this (pretty much a Striker that adds level to all damage, rather than +additional mod with a slight bonus or +1d6 per tier that other Strikers get). You wouldn't really have to change all that much, except for some of the powers for the roles (Defenders would likely need to get their encounters tweaked to be more control based than additional damage). As to the ability design, if you look at how the martial classes were built in the Essentials books, you've got a great model: there is a single encounter power that gets improvements while you level (increased damage die at certain levels, additional secondary effects with others). For each "class", you can create 2 encounters (each doing different things and progressing differently with level) and 1 daily (to represent the long CD), along with 2 at-wills (one to represent a basic attack and a second to represent a special, at will, attack). Furthermore, rather than using a downtime system, you could use a progressive recharge system where the player rolls a dice trying to get a specific number or higher (standard in 4e is rolling a d6 trying to get anywhere from a 4-6 depending upon the desired average use rate; the tweak would be modifying the dice roll be to a different dice, potentially, and providing a stacking bonus of +1-2 for every failed roll; the "encounters" could roll a d6 and require a 4+ whereas the "daily" could roll a d20 and require a 20+). Others could be predicated upon returning to base. For the combat system itself, you probably want to use a system without any attack rolls by default. The only rolls would be for specific mechanics: a passive that allows you to dodge basic attacks allows the defender to roll to avoid, a "skill shot" has to roll a d20 with specific listed chances (i.e. if it's a skill shot AoE cone, you roll a d20 for each target in the area and, on a 10+, you hit them; more powerful skill shots could require higher percentages), a passive that provides bonus crit damage would allow the user to roll to do said additional damage when making the attack. Mechanically, if you get rid of the tohit aspect, all you'd need to track would be hit points, bonuses to damage (split between magic and physical), and damage resistances (magic and physical) and simplify them by just making the bonuses/penalties flat reductions/increases. You could also include some interesting strategic benefits like floating bonuses to skill recharge rolls or flat bonuses to them. The only problem I could imagine would be the minions, since tracking that many tokens all the time could get *really* annoying, especially if they don't just die to a single hit (4e minion rule). All in all, you could do a surprising amount without all *that* much modification. Assuming the leveling/experience system wasn't too onerous it would play pretty efficiently too.
