Cycling calories so low

Try plugging your numbers into this website and I think you'll find 2000 to be much closer to the truth than 4000. I don't know all your variables, but on the data you've provided and assuming defaults for everything else it gives around 1590 calories for your ride. Since Garmin figures include BMR calories you might need to add another 280 calories or so for four hours riding, so 1870 total. Add a bit for wind and climbs and who knows?

http://bikecalculator.com/

If you upload your data to Strava, what sort of figures does it generate?

Also, if you use a power meter, as I do, calories should be calculated from your actual power and resulting work done, rather than vague things like heart rate and speed. Based on my experience I would also say that 2000 is a more expected figure for your ride than 3500 or 4000, assuming flat terrain and negligible wind.

Sorry I missed off it was 1100 m in elevation gain 

Strava just takes the calories from the edge - it doesn’t calculate them itself 

Using the calculator you suggest -  6200 calories ??

And haven’t bought power meters yet ! But will do .. 

You've input data in metric, but that means your energy is displayed in KJ, not calories. Switch metric/US units and you will see calories displayed.

I tried to find a similar ride from my own history, without a Garmin device. This is only 75km, but similar pace, recorded with my Wahoo ELEMNT. Elevation gain is a bit less per km, but it's 1733 Cals when I was probably around 85kg (not less, maybe more). Multiplying up to 100km would make it 2310 Cals with about 770m climbing.

Ah though I must be missing something ! Got it - thank you 

Also, the elevation field is not for elevation gain, it is for (average?) height above sea level to account for thinner air with less resistance, probably more relevant for time trialling at high speeds. What you need to use is the grade. 1100m in 100km, which is 1.1%.

With those adjustments I'm seeing 2565 Cals for your ride, so 2000 does seem low, but 3500-4000 is even further adrift.

Here’s the calories using that calculator - 1243

i find that hard to believe that 4 hours cycling only burbs that few calories ..