Leadership as a section would be a nice way to clearly demarcate your leadership achievements. Its a bold move. With 4 years of experience, if you dont have something really strong to showcase, people might end up comparing with strong candidates and you may loose there. This means you are clearly announcing that i am a leader and this what i have done. I would suggest to play a little mildly.

Resume is a document by design. You can alter the design, if you are allowed to, and make yourself look strong candidate. Do it by underpromise and indirect reference.

I can send a basic design if you tell me your choice of colleges.

There are some normal conventions for a resume but the resume can be tailored to capture your experience in the best manner. If your leadership is noteworthy having a section called Leadership is an excellent way to highlight this fact for the admissions committee so go for it.

You can have a "Leadership" section as long as it does not create any subject matter or chronological confusion with other sections. I would stick to a more traditional Extra Curricular/Other Activities type section and list your leadership achievements there. Or if some of them occurred during undergrad then you can always list them in that section. Again, as somebody suggested, if you are going to break with the typical mold, then your leadership experiences should be really stand out, and that does not mean Fraternity treasurer:)

I will also advise you to start off the bullet points of your experience descriptions with words like, "Lead," "Managed," "Designed," and so forth, strong action words that describe your leadership roles, ESPECIALLY when beyond the scope of being "just" a contributor in each position.

the most impressive leadership actions to mention in a resume will be
Born Leader but learnt it too
- always showcase that you were a born leader (subtly) the the situation also led you to be a expressed leader - you always set clear goals for your subordinates or whoever was under you

-Managed Motivation
showcase some example when you led people through trials and came up on the winning side by fighting with them.
show that as an excellent leader you used your relationships with team members to identify these individualized motivators

Created your team slowly but surely(my favourite for B Schools)
-this point has a point of demarcation, which means you did create but slowly. So here it will hit the reader as you had the eye for a fit and you always choose the best.

Lead and lead fiercely by example

Trust your team members : exactly the opposite of above point and requires delegation and hence trust. Example for a resume : find better ways to delegate and monitor progress more closely...this created a second bastion of leaders at lower levels.

Please add your leadership bit under your work experience section itself. Also instead of making a separate section be clear and explain your job role and responsibilities you undertook so your leadership experience is showcased well.