Two year olds and four year olds are both at stages where they are naturally inclined to test limits. It is a developmental stage we all go through intermittently while growing up. It is hard to keep our adult perspective when two children are going through this period at the same time ( while at different levels)
Keep in mind that 1. you are still the most important and beloved person in their lives and 2.they do need to know what limits are for their own safety and perspective in the world.
How to do this without getting all stressed out and starting to feel out of control ourselves is the challenge.
Some hints are that children actually seem to hear and understand better if you get down to their level physically and look closely into their eyes and speak so softly they need to work at being able to hear.
Try to divide and conquer and not deal with both at the same time but separate them and talk to each separately. Even though testing limits and boundaries is a common issue , since they are at different ages, the actual limits and boundaries may be different for each child.
I think they will settle down once they actually know what your limits and rules and expectations are and what the consequences are if they go beyond.
Also remember that time out is shorter for younger children even though they may seem to need more of a punishment. It should be a way of stopping and thinking about what has just happened not an actual punishment for bad behavior
Like always, offering alternate choices for the undesirable actions might help them learn something (which is after all the mutual goal you all have) so offering a plastic table cloth or drop cloth with paints ,water, smocks and big pieces of paper and supervision instead of blowing up when they decide to be creative and paint the walls with their muddy hands could be fun for all if mommy has the time to join in. But first the mess has to be cleaned up and it is ok to help them do this.
And if they can't go in the pool or to the beach, and you dont want to put up the wading pool , small tubs or pans of water and some kitchen tools or cups and spoons on the patio or even on a drop cloth on the kitchen floor could help get that water play need fulfilled.
Sometimes the four year old needs to have a space the two year old can't get into so structures can be made and not destroyed by little sibling.
Anticipating things starting to get out of hand and offering suggestions or finding out what each perceives as a need can help avert the situations where you feel so frustrated.
Good luck. This too shall pass... and blessings on the whole family!!
N.