In some messages, the syntax "$4" is not a variable name but a price in dollars (see for instance [[meta:Translations:FDC portal/Annual report on the Funds Dissemination Committee process 2012-2013/19/en]]), however it is presented in the Special:Translate interface as a suggestion of translation variable in the bottom of the translation window.
See Also:
T60746: [Insertables] Variable names containing hyphen are truncated
A possible resolution would be to send the translation variable names along with the original message in the Translate extension. I didn’t dive into the code but I guess the message is sent in a JSON variable, where an additionnal field could be added.
There are several wars to easily work around this. Add space between the dollar sign and number, don't use the dollar sign, add a zero width marker between the dollar sign and the number. Take your pick and be happy :).
My point: all these amounts, numbers, dates etc should be excluded from translatable text (moved into templates or hided tgrough translation variable).
For this example may be markup: <tvar|amount>$4</>