In theory, in terms of acne clearance, there should be no difference whatsoever.
However, the longer less-intensive treatment is a safer option if the issue of side-effects is a worry to you. I'm not a doctor (unsurprisingly) but there is no objectively 'better' option of the two. Which one you do would depend on a variety of factors such as your weight, acne severity and personal willingness to endure the worst of the 'likely' side-effects (i.e. intense dryness, aches, facial redness etc).
The cumulative dosage is really what matters when you are taking accutane. So in theory either of those treatments should be just as effective. Personally I would go for the 8 month course because after experience I was on 40 mg for 2 months and 60 mg for another 5 months and I didn't get clear until month 5. I would be worried if I was on a 4 month course that I wouldn't have enough time on the medication to get clear. After you go through the initial breakout and wait for your skin to stop purging you are usually into your 3rd month. That seems to be the norm with the accutane logs on here and with people I know who have taken the medication. So 4 months I would think just is not long enough of a course. Also according to my dermatologist you are supposed to stay on the medication until you don't experience a pimple in a month period and then she adds another month on top of that.
How much do you weight in kg and what is your acne severity/location?
I weight 90kg. The acne on my back was severe. The acne on my face was mild/moderate but scarred easily. I'm on my 5th month of Accutane. I still get pimples on my face but not on my back which is rather odd. The dosage is 60mg per day atm.
You are currently taking 0.66mg/kg/day (60mg/day).
Although sometimes derms go higher, the typical daily dose is 0.5-1.0mg/kg/day, which for you is 45-90mg/day.
Since you aren't clear yet despite being in your 5th month, you could ask your Derm to increase the dose to 1.0mg/kg/day (90mg/day) for the remainder of your course, assuming the side effects are tolerable.
Also, since the ideal cumulative dose is 120-150mg/kg to minimise the chance of relapse post-Accutane, you need to take 10,800mg-13,500mg of Accutane in total to maximise the chance of permanent remission.
Good luck