By 4 months, the swelling associated with the blepharoplasty should have resolved enough for your surgeon to accurately assess your ptosis.
I would recommend a consultation with an Oculofacial surgeon. I have posted a resource for you to find one near you in the link below
You should wait about 6 months to be sure that any swelling has gone away as this can make the lid droop. At 6 months, a good option is then a Mullers muscle resection to elevate the lid. This is a surgery that is done from the underside of the lid. It is a great surgery for patients who do not need any skin removed. See an oculoplastic surgeon so they can do a special test to see if you are a candidate for this surgery.
Under many insurance plans blepharoptosis repair is now bundled with blepharoplasty surgery meaning the insurance won't cover both simultaneously. This is unfortunate as it is medically and conveniently ideal to do them simultaneously. I often will do one or the other anytime after 3 months as long as their is no residual swelling from the initial surgery. Sometimes there can be some internal scarring, which is normal following an upper blepharoplasty that can make the ptosis repair a little more challenging. However, performing one surgery after the other had been done is generally not a big issue for experienced surgeons.
It is not routinely repaired. Sometimes swelling associated with the surgery can make the ptosis worse. Sometimes it is simply that the blepharoplasty was unmasked by the blepharoplasty. Generally by 4 months it is reasonable to have this surgically addressed. As a practical matter, you may that by the time you find the right surgeon and get scheduled, you will be close to 6 months after your surgery, a time frame that most surgeons are comfortable with.
If ptosis has been unmasked by blepharoplasty and you desire it to be corrected this can be done at any time.
Ptosis repair is normally done at the same time as upper blepharoplasty. Since you had the upper blepharoplasty without ptosis repair, you should discuss the timing of your ptosis repair with the surgeon who would be performing the procedure. Each surgeon may have a different preference as to how long to wait in between procedures. I would think that 6 months after your upper blepharoplasty would be fine, but there may be some surgeons who would perform the procedure now at 4 months after.