I play mainly kubicki's - as you've found the ex-D is awesome, and makes a 5 a lot less necessary. Over the years I tried a couple of 5's (very briefly), and never got on with them, and didn't need them. It was pretty disasterous - each hand on different strings!
Then my wife bought be a Cort Elrick 5, and as soon as I picked it up I could play it. I've found it handy on gigs where I have to read - particulary when the parts aren't written for bass. Theatrical parts end up in "non-guitar" keys, so having C and Eb available on the neck is a big help. I still use my Kubicki's for all of 99% of my gigs - they're what I'm used to, they're lighter and faster, and sound great, but I've had no problem switching back and forth.
I think the trick is the string spacing - the Elrick's are 19mm(ish). I've since added a Yamaha BBG5 - they make excellent cheap 5 strings, and tuned it EADGC. The Yamaha is my play around the house bass, and again it has 19mm spacing. I found the high C string easier to adjust to, as I just "forget" it's there when I'm not using it. Occasionally I got confused over the low B, but never had a problem getting my fingers in the right place.
If you're happy with your 4's then there's no reason to stop using them. Having a 5 in the bag when I need it isn't a bad idea.
Ian