I've done some web development work on my raspberry pi 4. It's almost there, but even using vim, some things are still too slow (I do primarily front-end development). 4gb of RAM means I can't have a ton of things going on, but it's easily enough for back-end, webpack, editor, and a handful of tabs without much issue.
Now, there are some CPU differences here. 3399 has 2 A72 cores at 2GHz and 4 A53 cores at 1.5GHz. The pi 4 can be overclocked to 1.8-2GHz pretty easily which makes it about as fast. I believe the Pi 4 GPU is slower, but it's mostly not relevant for dev work. When Ubuntu finishes working out the bugs with their 64-bit OS, it should offer another big speedup. That is probably enough to boost JS performance (and general system performance) high enough to not feel especially painful.
I would take 4xA72 over 2xA72+4xA53 any time. Heterogeneous systems are annoying. Linux probably has some fancy scheduler settings to prefer fast cores (??) but FreeBSD will happily schedule compile jobs onto the slow-ass A53 cores when the A72s are idle, which is annoying.
4xA72@2GHz is not bad for web development (I have that on my MACCHIATObin), the thing with the RPi4 is that it has worse RAM and I/O. SD cards are garbage and you really only have USB3 to share between everything else. Having native SATA makes everything better :)
There is only 1 PCIe lane and it goes to the USB3 controller. Running from a SSD on USB3 isn't that bad, but they still haven't made default booting from USB possible yet.
I don't think RAM is an issue though. Performance seems to scale pretty much linearly with clockspeed improvements which indicates either the RAM timing is tied to the clockspeeds (seems unlikely) so bandwidth is increasing or it already has more than it can use. That said, rk3399 designs tend to get about 20% better performance (or around 1gb/s) on synthetics.
I'm surprised better IO doesn't exist on the chip if Broadcomm planned on selling it to other customers. I don't think a single SATA port would greatly increase costs, but would have a huge impact on performance.
As a final thought, they really need to consider mounting the RAM and CPU on the other side of the PCB. That would allow good cooling solutions that don't compromise the other board functionality.
I use a few plugins to make web development a better experience. Nothing particularly exceptional. Syntax highlighting, nerdtree, ctr-p, linter, prettier, bracket highlighter, etc.
Now, there are some CPU differences here. 3399 has 2 A72 cores at 2GHz and 4 A53 cores at 1.5GHz. The pi 4 can be overclocked to 1.8-2GHz pretty easily which makes it about as fast. I believe the Pi 4 GPU is slower, but it's mostly not relevant for dev work. When Ubuntu finishes working out the bugs with their 64-bit OS, it should offer another big speedup. That is probably enough to boost JS performance (and general system performance) high enough to not feel especially painful.