The US Navy really wasn't all that interested in patrolling rivers, they felt it was the US Armies problem. But, when you get orders you say "Aye, aye, Sir!"
The first attempt (the PBR I, Aug 1965) was just civilian pleasure boat with guns. The Mk II (1967) was a slightly upgraded version of the Mk I. All Mk Is and all but 27 Mk IIs were transferred to Vietnam in 1973.
The PBR Mk III was developed in 1979 and featured an aluminum (vice fiberglass) hull and replaced the twin 50s of the Mk I & II with a 40mm.
http://www.warboats.org/mk3.htm
There is CONSIDERABLE confusion regarding the PBR Mk IV (or Mk VI depending on how you look at it). The US Navy replaced the "Patrol Boat River" with the "Patrol Craft Fast" (and re-designated existing PBRs to PCFs) in 1994. One's and two's of various designs were obtained (and evaluated), but there really was no "standard" Mk VI. Their primary function was SEAL support, but were used for riverine operations as well. SEALs traveled on any form of surface transport available, often resorting to taping lawn furniture to the deck of the available craft.
In 1995 the Navy adopted the "Special Operations Craft Mk V" which was specifically designed for SpecOps/SEAL support. These boats have a VERY limited patrol duration and they were intended as ship-to-shore transports not "Patrol Boats".
In 2014 the "Patrol Boat Mk VI" (A.K.A. the "SAFE Boat Mk VI") was adopted with a 5-day patrol duration and can be configured for either patrol duties or SEAL insertion.
So there was "officially" no PBR Mk IV. The "Patrol Boat Mk IV or VI" is "officially" the 4th in the series but it's also the 6th. As the "SOC Mk V" isn't "officially" a "Patrol Boat".
Confused yet?
And on that topic ...
Unit# 488 SPB Mk.III:
Weapon#1 57 HE=64 AP=16
Weapon#2 55 HE=90 AP=0
Use any LMB and Icon you like as these are non-standard boats.