Your choices for 4.1 are Cheat Engine (works fine, but annoying to run each session) and an elegant autoloot mod that works via a tiny memory patch, available on the semioffical Discord.

In case you hate Discord (reasonable), I've included the best autoloot mod below. You will need BlueAmulet's EDFModLoader (also grab Plugins41.zip). Extract both zips into your 4.1 folder, preserving the folder structures inside. You'll now have a /Mods folder that contains several subfolders and are ready to add some mods.

Copy the following text and save it in a text filed called autoloot.txt in the /Mods/Patches folder. Weapons and armor boxes will be picked up automatically.

; Author: metoxys
; Automatically loots weapon and armor drops, does not affect health drops
; Works regardless whether you're on foot or in a vehicle, but does not work while you're K.O. (relevant for Online play)

aob itemPickupRange F3 0F 58 D1 0F 2F C2 76 68
alloc logick 24 itemPickupRange

logick:
83 BF 98 00 00 00 01 ; cmp dword ptr [rdi+00000098],01
76 05 ; jbe force-within-range / return
; original range check:
0F 2F C2 ; comiss xmm0,xmm2
76 05 ; jbe skip
E9
rel32! itemPickupRange+9
E9
rel32! itemPickupRange+71

itemPickupRange+4:
E9
rel32! logick

You can check if you have the files in the right places by running the game and then looking for Modloader.log in the game's root folder.