How to know if its 3/4 or 4/4 Spirocore ?

Yeah, I went through this last year. 4/4s are longer, but unless you know the lengths or have both sets to compare, they are visually indistinguishable.

If your question is about a set of weichs that you have, I have an old set of 4/4 weichs that I could measure for you. I don't know if the lengths are consistent with mittels, though.

 

Yeah, let me go measure. 4/4 will be longer and I'm certain that mine are 4/4 because I went through this with these strings at the shop where we could also measure known varieties.

 

This may not be quite as helpful as I suggested, but here's what I've got for you:

I have a box of old strings and there were 8 Spirocore Weich in there. 4 of them were recently on my bass and when I was in Jake's shop, we compared their lengths to 3/4 and 4/4 Weich off of his shelf and easily determined that mine were 4/4s based on that comparison. I don't know for certain which 4 those were. I measured them all this morning and got some variance. You'll need more data to draw conclusions, but hopefully I'm a starting point. I'll also say that while old Spirocores are good, a few of these are well past any reasonable wear patterns and it was hard to straighten out the peg end to get a precise measurement. I never cut length off of strings, but it's possible that one or more of these came to me in a trade at some point in the past and I can't say they weren't cut by someone else. The 4 that were on my bass were not, but again, I'm not sure which 4 those are. That said, here's my results from this morning:

E String #1 and #2 were both 67 inches/~170cm
A String #1 and #2 were both 71 inches/180.34cm
D String #1 was 71 inches/180.34cm
D String #2 was 72 inches/~183cm
G String #1 was 70 inches/~178cm
G String #2 was 68.5 inches/~174cm

I hope that helps. I don't really understand why Thomastik doesn't make this easier on us.