I think it would be a stretch to use a blanket statement that "Yes, jellybean fixed the issue of lag". For one, I had awful lag on the Samsung Galaxy S3 while running Jellybean. And no, I am not blind either. I had a degree of lag on the Note 2 despite it being a powerhouse. While I had the Galaxy S4, also running Jellybean, after replacing the stock launcher with Nova I did not experience lag.
The other thing I have found with my personal experience with android devices is, after a period of time, the tweaks and customization would catch up with the device. The more 3rd party options I enabled, the more customization I did, the worse the lag (or introduction of lag if it did not previously exist). So even if a device I had was snappy and lag free on days 1-30.....that was not the case come day 120 necessarily.
I am not going to attempt to tell you your Nexus 4 has lag. I've never used it and it would be ignorant for me to tell you that you are wrong simply because of my experience. But to say that android no longer has lag with the issuance of, as they call it, project butter would be an inaccurate statement.