I have been talking to PI OLEDB Provider (Classic) developers and there is no padding / chopping implemented in any of the versions.
I have also installed both versions you mentioned and found that the behavior you describe for PI OLEDB Provider 184.108.40.206 is when casting to string. Version 220.127.116.11 is rounding to 4 decimal places. Recent versions do not round when casting to string. To be able to achieve a rounding / formatting the Format function was introduced (not supported in 18.104.22.168).
Let's assume your 3rd party applications query looks similar to this:
SELECT tag, time, cast(value AS String) value FROM picomp WHERE tag like 'SIN%' and time > '*-2h'
The equivalent using Format function would look like this:
SELECT tag, time, Format(value, '#.####') value FROM picomp WHERE tag like 'SIN%' and time > '*-2h'
I have passed this info on to 3rd party since the SQL is embedded in the code. Hopefully they can make use of this info to fix the problem.