We have a four syllable last name which I feel means I have to rule out all the lovely 3-4 syllable fn. what’s your opinion on this?
Would a combo of 2-3-1-4 sound odd to you?
As long as the rhythm works and it flows well, I don’t think it matters at all how many syllables the full name has. Besides, most of the time they’ll either be going by their first name (or nickname) or first/last, so it won’t seem “so much” in daily life.
It’s hard to imagine without putting actual names there, so that’s what I tried, and I think it sounds fine
Ex: [name]Bridget[/name] [name]Genevieve[/name] [name]Rose[/name] Eisenhower (only four-syllable last name I could think of off the top of my head! lol) I think that sounds really nice. [name]Bridget[/name] Eisenhower.
Ex: [name]Leo[/name] [name]Christopher[/name] [name]Blaise[/name] Eisenhower. [name]Leo[/name] Eisenhower.
All of this is pretty theoretical though, cuz a lot depends on the cadence of your last name if the accented syllables are different. These names might not work whereas other would. But I certainly think the combination of syllables sounds just fine.
Honestly, I don’t really think you need to rule out three-syllable names if there are some you really like. Four syllable fn might be pushing it, but again, I think there are some that would work. I know a girl with a three syllable first whose last name is a three-syllable mouthful and it actually sounds really good. (I know your last name is four but I think it could still be done.)
Ex: [name]Katherine[/name] Eisenhower, [name]Elizabeth[/name] Eisenhower (four and four!), [name]Felicity[/name] Eisenhower (four and four again), [name]Margaret[/name] Eisenhower, [name]William[/name] Eisenhower, [name]Theodore[/name] Eisenhower. I think all these could work. and you could always employ a shorter nn.
The number of syllables matters less than the rhythm of the name. Plus, like others have pointed out, your child will be know as FN+LN or even NN+LN, so as long as that part isn’t too much of a mouthful, I think you’re good.
I don’t think you need to let go of 3 or 4 syllable first names because of a long surname. I know a girl whose surname is 7 syllables (and 15 letters) and she’s done just fine with a 3 syllable first name 
It’s hard to say if a specific syllable count sounds okay to not- it’s really dependent on the names