SQL Server 2012 :: Best Way To Handle Like Percentage On Column Too Large For Index

We have a table to 100M rows and up until now we were fine with an non clustered index a varchar(4000) because we never went above 900 bytes (yes it is a bad design).We have the need to support international character sets now so the column was updated to nvarchar(4000) and now we have data past the 900 byte limit.

The data is long, seems useless but is needed by the business and they need to be able to search "where bigcolumn like 'test%'". With an index, even with a huge amount of data, it was 'fast'. Now of course without an index it is unusable. The wildcard is always at the end of the search. I made a full text index on the column and basic queries such as: select * from ourtable where contains(bigcolumn, 'AReallyLongStringofTextHere') works fine unless there is a space in the data. We loose thousands of returned rows because of spaces in the data.

I have tried select * from ourtable where contains(bigcolumn, '"AReallyLongStringofTextHere that includes spaces"') but not all of the data is returned. I get 112 rows with the contains statement. The table scanning statement of "select * from ourtable where bigcolumn like 'AReallyLongStringofTextHere that includes spaces%' returns 1939 rows.I understand that a full text index is breaking the long string up since it contains spaces. Is there a way to retain the entire string as 1 index entry or is there a way to fix my query to return all of the rows?