How to find previous of a value in one column looking from another column in Snowflake?

I have a table as below. How can I find the previous of each non null value in col5 looking from col4 in the most efficient manner using a SQL query in Snowflake?

col1   col2   col3   col4                       col5
aaa    bbb     -1    null                       2000-01-01 08:10:00.471
aaa    bbb     -1    null                       2000-01-01 08:09:55.678
aaa    bbb     -1    null                       2000-01-01 08:09:57.111
aaa    bbb     -1    null                       2000-01-01 08:11:15.564
aaa    bbb      0    2000-01-01 08:12:56.672    null
aaa    bbb      1    2000-01-01 08:09:00.897    null
aaa    bbb      2    2000-01-01 08:10:30.478    null

The result-set should be like below.

col1   col2   col3   col4                       col5                      col6
aaa    bbb     -1    null                       2000-01-01 08:10:00.471   2000-01-01 08:09:00.897
aaa    bbb     -1    null                       2000-01-01 08:09:55.678   2000-01-01 08:09:00.897
aaa    bbb     -1    null                       2000-01-01 08:09:57.111   2000-01-01 08:09:00.897
aaa    bbb     -1    null                       2000-01-01 08:11:15.564   2000-01-01 08:10:30.478
aaa    bbb      0    2000-01-01 08:12:56.672    null                      null
aaa    bbb      1    2000-01-01 08:09:00.897    null                      null
aaa    bbb      2    2000-01-01 08:10:30.478    null                      null

I have tried to use the lag and last_value functions so far but lag returns value from same column and last_value also didnt work.

Any help would be appreciated here.

From your question, it does not seem necessary to use window functions. Your explanations do not make it possible to understand how to use them.

Look at a simpler example.

select col1,col2,col3,col4,col5 
  ,(select max(col4) from test t2 where t2.col1=t.col1 and t2.col2=t.col2
      and col4<t.col5
  )new_col6
  ,test_col6
from test t

fiddle