I spooled up an UL with 4# yo-zuri hybrid last week before a week long trip. Brand new spool, just opened it up. Gave it the yank test and it was solid. Couldn't break it by hand without cutting myself. After 4 full days of fishing with that rod it started to get real weak. I was breaking off on every snag and the bigger fish when they got in the net. So I did the yank test and it broke easily. Kept peeling line 6' at a time and breaking it with ease. Got down way past line that I used and it still broke with ease. Put the scale on the end and gave it a pull, right at 4#. So it lost over half its breaking strength in maybe 30 hours of sun contact. Is that normal? I checked the spool (stored indoors) and it breaks right at 9# like advertised. Its not my rod nor my reel, its acting just like old line that sits in the sun for weeks. Real brittle. No frays or anything that I can see or feel. 

Wow... Sun is mighty powerful, plus chemicals in water ways can contribute... I never used less than 8 myself... I change out every 3 to 4 full days, for years fishing in brackish or salt everyday, I kid you not. Humm, but idk.. I have always noticed my co polys or whatever becoming slightly more brittle too... Why I change out so often. I just want fresh line on, just a old habit...

I have been using 6lb Yo Zuri Ultra Soft and never had an issue.  I can only guess that it is either bad line or something in the water caused it to breakdown quickly.   I have reel in the garage now that has had it on it for almost two years.  I may fish this set up once every two weeks but I have it out with me all day.  I do use the KVD line conditioner every once in awhile, maybe that helps with the sun. 

Never used 4lb, but have a 1lb spool of 6lb I have had for 3 years now.  No problems, but it has been stored correctly!

 

Jeff