#4 solid??

[kwired]

They have splices for this. It’s generally commercial or utility.
We have irreversible splices that are rated for direct buried also.
You have to hunt up the UL listed ones.

Here’s a butt splice.

4 AWG Non Insulated Butt Splice Connectors, 2 1/4" Length, UL Approved, 10 Pieces - BNT22-58

NON INSULATED BUTT SPLICE CONNECTORS Non Insulated butt splice connectors protect the electrical connections from wire pull-outs, abrasion and mechanical abuse. Butt splice connectors must be crimped on both ends to work effectively. For use with copper wires only. Dimensions & Features: Wire...

www.ferrulesdirect.com

There's others listed for grounding as well. Availability is the bigger issue for many.

 

[kwired]

310.3(C) Stranded Conductors.
Where installed in raceways, conductors 8 AWG and larger shall be stranded, unless specifically permitted or required elsewhere in this Code to be solid.

The typical short length straight down from an enclosure and ending not far below grade level is considered a sleeve and not a raceway. I guess the run to a water pipe or building steel can also be considered a sleeve in most cases. Solid conductor not going to be fun to pull if there are bends in the run though.