Hey RC
The stuff is definately weldable...if you follow a good weld procedure.
4130 is a heat-treatable low-alloy (HTLA) steel. It is highly susceptible to hydrogen cracking. Low hydrogen process, clean materials, low heat input, proper filler material and preheat must be used. Postweld heat treatment is usually done to bring the strength back up.
I wouldn't touch this job without a WPS. High pressure stuff + iffy weld procedure could lead to some serious damage to people and property.
You were right to look for more info.
Tyrone