Like many of you I was getting the prompt to update the Precise Fly Database.  I got a failure everytime I tried.
My setup is as follows:
- Spark
- RC controller
- OTG cable
- iPhone 5S or iPad 9.7
- DJI Go 4

I was able to download the update using either Cellular or WiFi with no issues (on either iPhone or iPad).  But when it hit the Updating phase it just sat there for 30 seconds and then gave the failure.  It appears it was trying to update the Spark as the indicators were blinking Yellow.  It seems like a timeout was happening.  I tried a dozen times with no luck.  I rebooted everything and tried again... no luck.  I tried switching OTG cables... no luck.

The final resolution was to disconnect the RC controller by doing the following:
- Remove props from Spark (safety first!)
- Make sure RC controller is turned off.
- Power on Spark by holding battery buttonfor a couple seconds.
- After Spark indicators blink Yellow, hold battery button for 3 seconds until you hear 1 beep.
- This will activate the WiFi on the Spark.

Next connect iPhone or iPad to Spark using Wifi
- Go to Settings > Wifi and select your Spark WiFi
- Enter the password found under your battery compartment (hopefully you wrote this down)... it is also on the box.
- You should now be connected to the Spark over Wifi.

Launch DJI Go 4
- You should get prompted to do update, and if so do the update.  
- If you don't get prompted then go into the Settings (3 dots in upper right), then choose 3 dots at bottom and scroll down to About and click Check for update.
- After update is complete, be sure to restart the Spark and restart DJI Go 4.
- Go back into DJI Go 4 and verify the update complete.

Reconnect the RC Controller
- Power on Spark by holding battery button for a couple seconds.
- After Spark indicators blink Yellow, hold battery button for 3 seconds until you hear 1 beep.
- Hold Pause + Function + Custom buttons on RC Controller at same time until you hear a quick beep followed by alternating beeps.  Let buttons go and wait a few seconds
- RC Controller should bind to Spark and the indicator light on RC will turn green.
- Turn Spark and RC off.
- Put props back on Spark

Hope this helps!

Also be sure to re-calibrate the Compass and IMU before you fly!

OTG is the issue.
The Precise Fly Database update is intended to WiFi to the controller and the controller sends it to the Spark.
I recommend OTG for flying only
But using OTG for any other use is asking for trouble.
There is no proof but I believe the bricked controllers from the last controller update 0.0600 was caused by OTG connections.

And most of all, do not re-calibrate IMU and Compass unless you have an issue or move flight locations more than 400 miles away.

Re-cal is one of the most over used unnessecary actions in flight preperations.