Try this:
|------------------------------1-3-4-----------------------------|
|------------------------1-3-4------1-3-4------------------------|
|------------------1-3-4------------------1-3-4------------------|
|------------1-3-4------------------------------1-3-4------------|
|------1-3-4------------------------------------------1-3-4------|
|1-3-4------------------------------------------------------1-3-4|
1st fret - pointer finger
3rd fret - ring finger
4th fret - pinky finger
|------------------------------1-2-4-----------------------------|
|------------------------1-2-4------1-2-4------------------------|
|------------------1-2-4------------------1-2-4------------------|
|------------1-2-4------------------------------1-2-4------------|
|------1-2-4------------------------------------------1-2-4------|
|1-2-4------------------------------------------------------1-2-4|
1st fret - pointer finger
2nd fret - ring finger
4th fret - pinky finger
By doing this it will strengthen your muscle memory, and then will be able to use your pinky in any situation, either coming after the middle finger or coming after the ring finger.
Another good exercise to do is:
|------------------------------1--4----------------------------|
|------------------------1--4------1--4------------------------|
|------------------1--4------------------1--4------------------|
|------------1--4------------------------------1--4------------|
|------1--4------------------------------------------1--4------|
|1--4------------------------------------------------------1--4|
1st fret - pointer finger
4th fret - pinky finger
|------------------------2-4------------------------|
|--------------------2-4-----2-4--------------------|
|----------------2-4-------------2-4----------------|
|-----------2-4----------------------2-4------------|
|------2-4-------------------------------2-4--------|
|-2-4----------------------------------------2-4----|
2nd fret - middle finger
4th fret - pinky finger
|------------------------3-4-----------------------|
|--------------------3-4-----3-4-------------------|
|----------------3-4-------------3-4---------------|
|-----------3-4---------------------3-4------------|
|------3-4------------------------------3-4--------|
|-3-4---------------------------------------3-4----|
3rd fret - ring finger
4th fret - pinky finger
Do these shapes all the way up the neck, and my friend, you will have a stronger pinky and better muscle memory.
Hope this helps! Any questions, message me!
-Jesse