I have not had my coffee yet but the gauge of wire in your y-splitter may be something to consider depending upon how many watts you might be pushing through it. Most of them I see are designed to split an input signal or for headphones. I'm sure they have some that are more heavy duty. Just a groggy thought.
Yup, 4 ohms...a Y is typically wired for parallel.
But, like previously stated, make sure the "Y" adapter cable is made with unshielded cable with the individual wires at least as heavy as typical lamp zip cord. You can see this if the Y's plug shells screw off. If the Y's plugs are molded it would be difficult to tell what type of wire is used on the inside...so, in that case, I wouldn't risk my amp with it.
Best to have your local amp tech or sound & light shop make you a Y cable out of speaker wire and switchcraft hardware, 1 male and 2 female ends. Then just use regular speaker cables to connect the cabs.
Series Y cables aka "triangle Y" are not common, you would have to go looking for one and chances are nobody you ask will have one. 99.99% chance your Y cable is parallel.
plug the speakers into the y cable and check the plug end of the cable with a meter. It should read roughly 3.2 ohms, give or take. somewhere between 2.9 and 4.
I have never heard of a Y cable that was wired series, but you never know, if it is wired series you will likely get a reading of between 14 and 15.5 ohms.
There are a few out there, but they are very hard to find. I found a couple once last year, but I can't find them now. It's easier to find a box that switches between series and parallel. I went looking for the series cable a couple months ago, couldn't find it, and ended up making a box myself.