which pilot would be better for a 4x crisis decay build?

Hello Small. Interesting question there.
So, Otto increases the fire rate of his weapons (35%), but not the reload time, so he's really good for magnums and spears, but the benefits are less on the radiation weapons. Unloading superfast is useful, but you very quickly have an empty gun.
Gintare's 15% boost was designed for the Reapers (I think), but she'll boost those Decays just fine. I guess it kinda depends on your playstyle in the Crisis. If you like to stay in cover, she might be the best pick, but the Decays suggest you might be inclined to push. You're always going to be 'hit and run' in the Crisis, and a target for the runners, but if you're careful, that can be a lethal build.
There's a lot of good War Robots brains in here. I hope they can give you something more definitive.

Sounds reasonable to me, and yes, it's a glass cannon so you'll need to stay on your toes. Maybe try them both, without too much levelling. You get the special ability straight away, so you can at least get a feel.

here's an Adrian vid from a year ago - 
2 hour livestream record, but he's got some good advice before the gaming. If you skip to 1:31 (ish) you'll get his assessment

The math on Otto’s benefit to Rads has been done:



For the longest time, we thought that Otto was only worth it on spear and magnum builds, but in reality, the Rad and Redeemer can also benefit from him somewhat, just a good bit less. A 17% burst DPS increase is nothing to scoff at; it’s a bit more than Gintare.

It has the benefits of not requiring you to expose yourself and fire for as long as well, but the downside is if you only expose yourself and fire off one burst, while that burst will come out faster, it will deal less total damage when compared to Gintare. Now if you continually fire, you’ll find that Otto is a bit better since his ability not rely on Crisis’ ability. Otto gives the crisis players options depending on the scenario; you can take a less risky playstyle and fire off just one burst and then go back to stealth. In this case, your stealth will be back a bit faster and you can run to cover a bit sooner. Or, you can take a more risky playstyle and use Otto’s permanent buff uptime to continue firing, but for a Crisis to remain exposed for that long… it’s dangerous, and you should only do it when firing on an enemy who you’re out ranging.

It’s also worth considering that Gintare will always be good on Crisis, but Otto will stop being useful if you switch off the Decays. If you ever plan to run something like Gauss Crisis (my personal favorite build), Gintare will work but Otto will not. Personally, I don't think the increase in the speed that that you unload your mag is worth the reduction in total mag damage, nor is the price of a pilot who only works on one build worth it. Stick with Gintare.