- Years
- 2011–about 2016
- Factory code
- To verify. Not in the two-stroke VIN table.
- Engine
- 124.2 cc DOHC four-valve four-stroke, fuel injection, derived from the Derbi GPR125
- Fuel system
- Fuel injection. No carburettor.
- Ignition
- Not described in the brief beyond fuel injection.
- Bodywork
- RSV4 styling.
- Power
- Around 15 hp (11 kW). A1-legal from new. No full-power version in the brief.
- Restricted and open
- There is no full-power version to unlock. The bike is described as A1-legal from new.
Key changes
- The two-stroke ends. This is a 124.2 cc DOHC four-valve four-stroke with fuel injection.
- The engine is derived from the Derbi GPR125. It is not the RS50’s D50B0, and it is not a Rotax 122 or 123.
- RSV4-styled bodywork. Around 15 hp, A1-legal from new, with no full-power version recorded.
- The factory code is To verify.
Common problems
- To verify. The brief has no owner-forum fault list for the RS4.
Special editions and replicas
Per Hartrusion, as recorded in the research brief, “Replica” means the decal scheme. The mechanical bike is the standard model of that year and code.
| Livery | Years | Status |
|---|---|---|
| Team Aprilia Alitalia Racing replica | RS4 125 span | Listed in the generation record. Exact years to verify. |
| Biaggi Racing replica | RS4 125 span | Listed in the generation record. Exact years to verify. |
The full list, including schemes that are not verified, is on the generations hub.
The RS4 is where the name stops meaning a Rotax two-stroke. From 2011 to about 2016 the sports 125 is a four-stroke: 124.2 cc, double overhead cam, four valves, fuel injection, and the brief says the engine is derived from the Derbi GPR125. The body is RSV4 styling. Power is given as around 15 hp, which the brief treats as 11 kW, A1-legal from new. There is no full-power version to “open up”. Anyone selling a derestriction kit for one is selling something this research does not support.
Do not order the wrong engine’s parts
Two confusions are worth writing down. The Derbi D50B0 is the RS50 engine from 2006 on, not an RS125 engine. Pedparts’ RS50 page is in the sources for that reason. And nothing from the Rotax 122 or 123 — reed block, powervalve, expansion chamber, PHBH, VHST — belongs on this bike. The parts directory keeps four-stroke service items in their own category, with empty OEM numbers.
The factory code is To verify. The ZD4 model codes in the brief stop at RD and RM. Do not force an RS4 VIN through the two-stroke code list and pretend the letters mean PY or RM. The decoder will show an unknown model code rather than guess.
What is not known yet
Common faults are To verify. The brief says so in those words: no owner-forum fault list had been researched. Valve clearances, plug grade, service intervals and torque are the same. The two-stroke MCN intervals (rings at 5,500 miles, a 60°C warm-up, premix-style two-stroke oil) do not transfer to a four-stroke. They are not printed on this page as if they did.
The year the four-stroke family took the plain “RS 125” name back is also disputed. Sources say 2016, 2017 or 2019. This page keeps the RS4 title through about 2016 and sends the renamed bike to the next generation, with that dispute repeated there.
Liveries
The generation record names a Team Aprilia Alitalia Racing replica and a Biaggi Racing replica. Exact years inside the RS4 span are To verify. As with the two-strokes, a replica is a decal scheme unless a source says otherwise. No source here says the RS4 replicas were mechanically different.
Sources
- Aprilia RS125 (opens in a new tab) — Wikipedia
- RS50 D50B engine (opens in a new tab) — Pedparts
- Aprilia model list (opens in a new tab) — Motorcyclespecs
Research brief dated 24 October 2026. Where this page says “the brief”, that is the sourced master plan behind the first edition.