Find how much younger is the new member

let the members be a1,b,c,d,e,f,g,h,i,j .Let a2 be the younger member who replaces a1.
we have,
    a1+b+c+......+j=a2+b+4+c+4+d+4+.....+j+4
=> a1+b+c+....+j=a2+b+c+..+j+36
hence,the oldest member is 36 years older than the new member

Let 'O' be the old persons' age, and 'N' be the new persons' age and 9X be the age of rest of 9 persons.
then, (9X+N)/10=(9X+O-10*4)     (10*4=> coz 4 yrs back for 10 persons)
=> 9X+N=9X-40
=>N=-40 (which means the new person is 40 years younger than the old person).
 and it solves the problem!!!

Let the old member be x
Remaining member be y
New member be z
Average ages be a

(x+9y)/10=a; --------------------(1)
4 years later
(9y +z+36)/10=a--------------(2)  //36 because collective ages of 9 person 4 years after
solving 1 and 2 you get

You get z+36=x
This means x is 36 year older then z

After four year increase in age is 36 years (one person is left). To get same average, the new member must be 36 years younger than the old member

It is hard to say that it could be 36 or 40 since we dont have data. as per the give calucalation, we reach only one statement. i.e, the age difference is a1=(4+a2)

age of committe members= a1+b+c+d+e+f+g+h+i+j+(10*4)
age of committe members with new member=a2+b+c+d+e+f+g+h+i+j+(9*4)
i.e, => a1+b+c+d+e+f+g+h+i+j+(10*4)=a2+b+c+d+e+f+g+h+i+j+(9*4)
i.e, => a1+b+c+d+e+f+g+h+i+j+(40)=a2+b+c+d+e+f+g+h+i+j+(36)
i.e=> a1+40=a2+36
i.e=>a1-a2=40-36=4
i.e=> a1=4+a2

Therefore, we can conclude that the new member is 4yrs younger

Four years back, Let the sum of the Ages of the 9 members of the committee( Who still continues )  be M. Let the age of the 10th memeber at that time be R.  Hence the average age of the 10 members was  (  M + R) / 10.   

Now the 10th member is  replaced with 11th member , whose current age is N. 

The current average age of the committee is ( M +(9X4) +N )/10 

Hence , (  M + R) / 10  =  ( M +36 +N )/10 

Hence R = 36 +N.

Thus, the new member is 36 years younger to the old member.

On the inclusion of the new member in the committee, there is no change in the average age of the committee members as it was 4 years back implying:

The newly substitute in the committee is a younger candidate who reduces the average age to what it was 4 years back quantfying to 4*10 total decrease in the total age i.e. 40 years.

How ever since the new candidate replaces on of the older employee the resulting decrease will be only 36 years(9*4) and not for 10 persons...

Hence the younger member is 36 years younger.