Is it possible to begin a movement action (Let's say my Warlock has move 5), move 3 of those spaces, take a standard action and then take the remaining 2 spaces of his movement action in the same turn?
Or is it illegal to begin one action, take another and then finish the previous one?
No, you cannot start one action, perform a second action, then complete the first action - each action is a discrete portion of time that can't be mixed around like that.
While there is no explicit rule stating that you cannot mix actions, there are specific powers that imply that each action must be performed before starting another action.
A prime example of this is the 13th level Monk Full Discipline power Dance of the Stinging Hornet (Full Discipline powers are - if both techniques are used - effectively the same as any other character using a move action encounter power and a standard action encounter power in the same round - the Monk expends two actions but only has to use one power to gain the benefits on both actions). The movement technique of this power is as follows:
Effect: You make an Athletics check to jump with a +5 power bonus. You are considered to have a running start. You can use the attack technique at any point during this movement.
A number of other Monk powers offer similar functionality by allowing them some type of movement and the use of the associated attack technique at any point during that movement.
It is also worth noting that across the entire range of official 4e books there has never been a power published that states something along the lines of "attacks made during this movement gain x" without also stating that the character gains a specific attack from the power.
These could be considered specific cases of granting attacks during movement, which would imply that it is otherwise not possible.
Some GM's however do allow free actions (or potentially a subset of available free actions) to be taken essentially at the same time as other actions, because oftentimes they have no direct consequence on how a turn plays out (such as yelling something to an ally).
Nature's Wrath
As a warden, you use Nature's Wrath to mark foes. Here are the things you should know about this fundamental class feature:
- It's a free action. One of the things this means is that you can use Nature's Wrath at any point in your turn, including midway through another action such as a move or charge. Another is that you can take a full turn's worth of standard, move, and minor actions without giving up your ability to mark.
I cannot find any other reference in the rules to using one action in the middle of another (with the exception of Immediate Interrupts in the middle of a triggering action), but the Warden Essentials article from Dragon 379 claims you can use Free actions in the middle of other actions.
To me, this explicit mention in an article about tricks when playing Wardens suggests that this behavior (using one action in the middle of another) is not normally permitted: it is a special feature of Free actions.
I don't have my book in front of me so I can't quote exact rules but 4th edition basically runs along the line of one action has to be resolved before you can take another one. So if you use your move action, or even part of it, and you decide to stop and take another action (I.E. standard to attack or minor to open a door), your move action for the turn is spent and you can't continue unless you substitute another action or a power allows you to, but even then it's counted as a second action and not part of the first.
This is also true for other actions.
If a monster has an ability that allows it to shift out of range when it's targeted by an attack, you can't then use your move action to move back into range and continue the attack.
The rules don't allow it but as a GM, I do when it makes sense. There is no good reason which utterly prevents you from shooting arrows while running. One task needs your feed and the other the arms -> no physical reason to forbid this.
But the multitasking comes at a cost. So when a player does this, I put a modifier on both actions (like -1 for both of you're running over a plain floor, -2 if you're in a rough street, -3 if you try this in field of shrubs and -4 for dense forest - you get the idea).
I find that this adds color to the game at little cost (uhm ... unless you have power gamers in your group ... then it will lead to endless discussions ... but that may be fun, too ... I guess ...). I even allow players to train such skills.
PS: In the real world, this is sport called "run archery" (http://en.wikipedia.org/wiki/Run_archery) and if you doubt that this can even work, watch this video: https://www.youtube.com/watch?v=2zGnxeSbb3g
