I am 46 and planning to retire in 3-4 years. Would it be better to purchase a personal health insurance policy now in addition to the employer-provided group health insurance policy? Or shall I purchase it post-retirement?
Yes, you can have an additional health insurance policy. If you buy a health policy and make no claim for two-three years, the coverage value increases over time. You will also be able to build a track record with the insurance company regarding your health record and the claims - whether or not you have availed them.
If you buy a health insurance policy, you will not have any significant cost advantage because health insurance pricing entirely depends on or gets revised every year. There is no advantage of buying it later.