In 4 nights you can go to Arenal and Monteverde areas, 2 nights each. But it really depends on what you are looking to do there. In 1 day in Arenal you can do 1 day trip or 2 activities that are in the area. In 2 or more days you can do more trips and/or activities. I think you can stay there for a week and not get bored. In 1 day in Monteverde you can hike and do a zipline. In 2 days in Monteverde you can do a lot more hiking. So it really depends on what you are looking to do. And remember travelling to each place takes up a half day at least.
So, Arenal and/or Monteverde-Montezuma-Manuel Antonio. You can drive to all those places. The drive to Arenal is much easier and faster from San Jose airport than Liberia. The drive to Monteverde (if you don't go to Arenal) is probably a little easier from San Jose also. Manuel Antonio is definitely closer to San Jose, not Liberia. So, looks like you should be flying into and out of San Jose.
To get to Montezuma from Arenal or Monteverde I would suggest you take the ferry. But it's not as you said "drive instead of taking the ferry". It's a longer drive on more of bad unpaved roads vs. drive, then ferry, then drive on some unpaved roads. Take the ferry to Paquera not Naranjo. Make sure to check the ferry schedule online and be in Puntarenas early.
If you don't want to drive you can take shuttles everywhere. Or, you can go from San Jose to Arenal via a white water rafting trip. You can go from Arenal to Monteverde via jeep-boat-jeep (saves an hour or two off driving time) or jeep-boat-horseback (which you wouldn't want to do if it's raining).
I don't understand Matt_1973's suggestion ("Why not see Arenal/Monteverde via Interbus..pick up your car in Liberia and then make your way down the peninsula maybe stopping off at some of the other beaches?"). Why would you waste time on Interbus from Arenal or Monteverde to Liberia and then drive back south? That could be 2 or 3 wasted hours. This plan only makes sense if you are leasurely driving to different beaches and staying there overnight, since getting from the main road to each different beach area takes about an hour. For someone who is just trying to get to Montezuma this plan makes no sense.