Sorry to answer you question with several questions, but "best" is hard to determine without more requirements.
What kind of fiber is in place and what are it's specs? 4km on single mode good quality fiber is pretty easy and reliable. 4km on multimode, not so good. What color/frequency of light are you wanting to use? 850nm, 1300nm, 1310nm or 1550nm or do you care? How many fiber links are being used and do they need to share the fiber path (Coarse Wave Division Multiplexing -CWDM?)
100Mbps or 1Gbps link? I would normally use only those units designed for large temperature swings and lack of environmental controls if they are outdoors in an uncooled enclosure.
-20C-+50C will cover most places, but not all...
Is a DIN rail mounting required?
What kind of power is availble?
Do you need Class 1 Div 2 certification or higher?
It sometimes requires a "managed" media converter so the performance can be monitored. Most of the network switches have fiber SFP ports that allow for better control and monitoring of the health of the fiber links though many are less heat tolerant that the media converters.
Here is a 100M unit as an example from Transition networks that offers extended temperature ranges (-40°C to 75°C):
Kind of like the "best" car... It mostly depends on your requirements... I would trim any vendors temperature specs by +/-10C just to be safe...
Basically, the more complicated the device and the newer the technology, the less heat tolerant it will generally be for the same dollars spent...
I am not sure what you are after exactly but these are some of the questions I would start off with...
Hope this helps,