Trust the training plan or go with my gut?

Relative to something like TrainerRoad, the Zwift plans are fairly straightforward. But in any case, I'd try holding my ramp test FTP for at least 45 minutes before putting it up. 
 

Personally, I don't like ramp tests (for a variety of reasons), and would argue for a TTE threshold ride, but that's a separate subject.

I'm on a 16 week programme which is currently less intense than my normal riding.  Four weeks in, I've spent the last month pootling around in HR Z2-3 in 3'C....freezing my arse off (I actually bought a warmer jacket).  

I can see looking at my calendar that it will pick up soon.  I did a road.cc one a few years ago which culminated in me doing a New Forest Sportive at 36kph (okay, flat and in perfect conditions, but my cycling did improve quite a bit).

Another thing to bear in mind is that you might have different abilities at different intensities; GCN did a good video testing an MTB rider vs a roadie, the MTB rider had a great VO2max and sprint, but an indifferent FTP per the 20min test (possibly because of poor pacing). As such, the coach said if he based his training zones on FTP he would under-train. 
 

The advantage of the ramp test is it doesn't need to be paced. 

Generally no harm nudging the intensity up or down a little if it feels off. FTP should only really be a baseline guide anyway - need to find what works for you, given your personal abilities, and taking into account your current health, restedness, the time of day, etc. That might mean nudging it up on certain types of session, and down on others. Some experimentation to learn what works is a good thing.

How long have you supposedly got until you are scheduled to test again?

If its only a couple of weeks, then I'd just stick to the plan.

Until recently, I'd have said to 'chase the ace' and always be pushing, but now, I'm coming round to the idea that the biggest overall, and long term gains are achieved by playing the long game. If you look at the programme, I guess load is increased either in increased intensity, or duration of effort as the weeks pass. So even if your FTP has moved on, you are still, in effect pushing your body.

At the start of the programme, you'll have been hitting intervals at the top of your range, and holding them for a shorter period. Now you are hitting the intervals at a likely lower percentage of your range, but holding the effort for longer. 

This will be asking your body questions in different ways, but it still is asking questions. That's no bad thing as it will limit the chances of burnout.

In extreme cases I've seen people stop striving for improvement as the numbers have got so big and efforts so demanding, that the fun is sucked out of it. 

So, if its only for another couple of weeks, I'd say enjoy some time not hanging out of your arse... there will be plenty of time to chew the bars after the next scheduled test. 

I had, pretty much, the exact same dilemma.

I ended up manually increasing my FTP on Zwift until the sessions felt hard enough again.

When I retested a bit later my new FTP was slightly higher than my guesstimate. I think if I had continued on the lower FTP I would not have got as much benefit.

I find that a ramp test doesn't take much out of me so alternatively you could just do that on a rest day?

I think it depends on the session; if it were a sweetspot/ftp session I'd probably leave it as is.  For a VO2max session I might tweak the target so that it gives the right amount of stress, if I didn't feel I was being pushed hard enough (bearing in mind what can seem ok in the early intervals can be very testing at the end of a session).

Why not do a ramp test?... perhaps you tested a bit low the last time if you weren't rested, or paced the 20 min test wrong.  If you are getting your FTP from the average of Zwift events then there is the possibility that it is being understated due to drafting etc.  If you want to get a more reliable FTP from an event, try an ITT on Tempus Fugit.