Today's threshold difficulty at 3,9 what would a miner need to go solo What i am asking is a relative threshold on the minimum Hash rate needed to mine solo, taking into account todays difficulty,could it be an ASIC of 1TH/s or 5TH/s so as to compare with pool mining
There is no minimum power needed to mine solo. You should note, however, that if you mine solo, your rewards will be much more inconsistent than if you mine in a pool. This is the major reason why pooled mining exists.

With 5 TH/s at the current difficulty, the average time for you to find a block is 38 days; with 1 TH/s, 189 days. But this is a random process that would average to 38 (or 189) days, and that time period is only likely to grow larger as time goes on. You have a real chance, then, of never getting any reward for your work (the reverse is also true, of course: you have a chance of finding more than expected...but I wouldn't stake thousands of dollars on it). Personally, I'd want an expected period of no more than 10 days/block (~20 TH/s currently) to consider solo mining a good option.

Also of note: your intuition of how any random process will work out is very probably very wrong. I'd go for mining in a pool so that I don't have to deal with any large amount of randomness in this process.