Can I use 4ml of 5% minoxdil per day (2ml twice a day)? Because 2ml per day can't cover my balding area.

In my opinion, I would first try to to cover your affected areas once a day or divide the affected areas and treat each one daily. This will allow you to keep  to  the total daily  recommended dosage. Minoxidil has potential to develop several cardiovascular and other side effects when overused. Consider adding another FDA approved treatment such as finasteride  or low level laser. Work with a hair loss expert on a treatment plan. PRP hair md.com

While minoxidil works for some, the
problem with the medicine is that it does not stop hair loss, and the
DHT will continue is binding to the hair shaft which will eventually
overtake any additional growth caused by the Minoxidil and die, perhaps
just slightly later than it would have without the medicine.

Also, the alcohol base required to support the active ingredients also cause irritation of
the scalp and sever reactions for many people.
The best thing to do would be to see a doctor who specializes in
hair regrowth and the rebalancing of body biochemistry, and to look into
a personalized medicine. Any pills that actually can make changes
inside your body, are required under the FDA to be available by
prescription only. This therefore means that it is not possible for
anything that does not require a prescription to cure or prevent hair
loss or regrow thinning hair.

The short answer is no. The dosings of medications have been carefully worked out to balance efficacy with safety. If you feel you need more, speak to your physician or pharmacist regarding how to apply minoxidil efficiently. If you feel thsi dose does not work, you might consider a variety of off label means but only under supervision. This could include using a bit more, or using oral minoxidil at low doses. The side effects of overdosing on minoxidil include worsening headaches, dizzininess, low blood pressure, heart palpitations, heart rhythm disturbances, ankle swelling, hair growth on the body. Minoxidil is a drug and use must be respected.

You can use a greater valume of minoxidil to cover the balding area and use it twice a day for best results