A dirty rad will definitely not help but 4 bars at idle is very not normal for a Vee. I've only ever seen 4 bars once and that was crawling in traffic in a 40 degree heat wave and even then that was with a few problems of my own which I've now resolved and I've not seen 4 bars again.
The fan should kick in after 3 bars have appeared but before 4, the fan should be able to hold the bike at 3 bars when idling in all but the most severe of ambient temperatures.
There are a few things for you to check really; (in order if difficulty)
- Is the thermostat opening properly - You can check this by seeing if the large radiator hoses/radiator body are getting hot.
- Is the fan buggered - Does it kick in sometime after 3 bars appear but before you see 4. If it doesn't kick in then it could be either the fan itself or the thermo-switch that tells it when to kick in.
- You can check the fan by applying 12v directly to it and seeing if it spins. Easy to do but the access is terrible and you'll probably be removing the radiator at this point.
* If the fan is good then you'll need to check the thermo-switch
- Check the thermo-switch as outlined in the service manual located here
https://www.v-strom.co.uk/downloads.htm, either K2 or the KLV1000 K4 will be fine. The thermo-switch is screwed into the back of the radiator so you're committing to rad off and coolant swap here.
- Is the coolant temperature sensor giving a proper reading - This is also a voltage check and the coolant temp sensor is buried between the V of the engine, screwed into the thermostat casing I believe.
Removing the radiator and giving it a good flush out will allow you to access and check the fan as well as ensuring the radiator core isn't clogged and preventing coolant from circulating. Cleaning the fins is easier with it off the bike as well, an airline is a nice help here if you can find one. As Gert mentions, the fins are super fragile, trying to straighten bent ones usually results in the very front part snapping off but I figured this was preferable to the entire channel being blocked. Just take it super careful.
One last thing to check if you have the radiator off is the shape of the plastic shroud on the rear. Mine had become pretty warped inwards at the sides and was fouling the fan as well as restricting the airflow. I used a heat gun on the corners to reform the guard and I've not had any issue with the fan or the airflow since.
Happy hunting