THe Elwro 800-4 Junior PC was an Elwro 800-3 Junior with new capabilities. It no longer had the toy piano housing, but a specially designed case that borrows elements from the Commodore 65.

The main differences to the previous version of the Elwro Junior were

• Built in 720k 3.5" Disk-Drive
• Default boot to CP/M
• Color TV modulator
• Support for 3 floppy disk drives

One of the biggest draw-back of this computer was the slower emulation for Spectrum Software: spectrum programs ran about 20% slower than on previous Elwro Juniors.

From 1985 to 1990 Elwro produced the Elwro 800 Junior line of computers, meant for education. The polish department of education needed computers for the students. Many biggers schools had opted for ZX Spectrum, Timex or MSX. For smaller schools, the computer had to be cheap, ZX Spectrum compatible and easy to produce and repair.

The 800 Junior computers were produced using an existing case that was designed for the "Elwirka" toy piano. The elwro computers therefore have an odd feature: a sheet music stand that folds up to hold a paper with a music score.

Elwro Junior computers were cheap, but well designed. It was ZX SPectrum compatible, but had better specifications than the Spectrum itself.

• Joystick connector
• SPectrum network connector
• Composite Output
• RGB Output
• Tape connector
• Junet network connector
• Diskdrive connector (if controller installed)
• Centronics Printer Port

JUNET was a serial bus network. The computers were daisy-chained; if you disconnected one computer, all computers further in the chain were disconnected from network. Connecting to network was performed by plugging 2-DIN plugs into the computer. Sockets are jumpered 1:1 inside. Network has 3 lines: Data, Clock and busy. The clock was set at 14MHz/256, with the master computer giving the clock to the rest; the data transfer rate is around 55kbit/sec The networking is done by 8521 USART or it's Soviet cousin, the KP580BB51A.

The Elwro company was established in February of 1959 in Wroclaw, Poland. The company's full name is The Wroclaw Electronic Works (Wroclawskie Zaklady Elektroniczne). The Elwro's first computer was the vacuum-tube based Odra 1001. This computer only reached the research stage and not mass produced. The next model was the Odra 1002 which replaced the vacuum tubes with transistors. The company also produced the tube-based UMC-1 from 1962 to 1965, 25 units were built.

The Z80 quickly became popular in the personal computer market, with many early personal computers, such as the TRS-80 and Sinclair ZX80, using the Z80 as their central processing unit (CPU). It was also widely used in home computers, such as the MSX range, SORD, and the Amstrad CPC, as well as in many arcade games. Additionally, it was also used in other applications such as industrial control systems, and embedded systems. The Z80 was widely used until the mid-1980s, when it was gradually replaced by newer microprocessors such as the Intel 80286 and the Motorola 68000.

The Z80 microprocessor was developed by Zilog, a company founded by Federico Faggin in 1974. The Z80 was released in July 1976, as a successor to the Intel 8080. It was designed to be fully compatible with the 8080, but also included new features such as an improved instruction set, more powerful interrupts, and a more sophisticated memory management system.

The Z80 quickly became popular in the personal computer market, with many early personal computers, such as the TRS-80 and Sinclair ZX80, using the Z80 as their central processing unit (CPU). It was also widely used in home computers, such as the MSX range, SORD, and the Amstrad CPC, as well as in many arcade games. Additionally, it was also used in other applications such as industrial control systems, and embedded systems. The Z80 was widely used until the mid-1980s, when it was gradually replaced by newer microprocessors such as the Intel 80286 and the Motorola 68000. The design was licensed to Synertek and Mostek as well as the European SGS.

The Z80s instruction set is binary compatible with the Intel 8080, so that 8080 code such as the CP/M Operating System and Intel's PL/M compiler for the 8080 can run unmodified on the Z80. The Z80 had many enhancements over the 8080 such as 16-bit data movement instructions, block copy and block I/O instructions, single bit addressing of all registers, IX/IY offset registers, better interrupt system and a complete duplicate register file for context switching during an interrupt.