diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index 1b6252b4..837ef8bb 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -52,9 +52,22 @@ target_link_libraries(
   GTest::gtest_main
 )
 
+# test compressing 4 channels (library API tests)
+add_executable(
+  test_4ch_codestream
+  test_4ch_codestream.cpp
+)
+
+target_link_libraries(
+  test_4ch_codestream
+  openjph
+  GTest::gtest_main
+)
+
 include(GoogleTest)
 gtest_add_tests(TARGET test_executables)
 gtest_add_tests(TARGET test_mixed_coc)
+gtest_add_tests(TARGET test_4ch_codestream)
 
 if (MSVC)
   add_custom_command(TARGET test_executables POST_BUILD
diff --git a/tests/test_4ch_codestream.cpp b/tests/test_4ch_codestream.cpp
new file mode 100644
index 00000000..f31bfd54
--- /dev/null
+++ b/tests/test_4ch_codestream.cpp
@@ -0,0 +1,119 @@
+//***************************************************************************/
+// This software is released under the 2-Clause BSD license, included
+// below.
+//
+// Copyright (c) 2019, Aous Naman
+// Copyright (c) 2019, Kakadu Software Pty Ltd, Australia
+// Copyright (c) 2019, The University of New South Wales, Australia
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+// 1. Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//
+// 2. Redistributions in binary form must reproduce the above copyright
+// notice, this list of conditions and the following disclaimer in the
+// documentation and/or other materials provided with the distribution.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS
+// IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
+// TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A
+// PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED
+// TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
+// PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
+// LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
+// NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
+// SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+//***************************************************************************/
+// This file is part of the OpenJPH software implementation.
+// File: test_codestream_api.cpp
+// Author: Pierre-Anthony Lemieux
+// Date: 11 May 2026
+//***************************************************************************/
+
+#include "gtest/gtest.h"
+
+#include "ojph_mem.h"
+#include "ojph_file.h"
+#include "ojph_codestream.h"
+#include "ojph_params.h"
+
+// Compress a 128x128 image with 4 components (3x 8-bit unsigned, 1x 32-bit
+// unsigned) using irreversible coding with qstep=0.0003.
+//
+// The exchange lines returned by codestream::exchange() are always si32-typed
+// (LFT_32BIT | LFT_INTEGER). For irreversible paths the library internally
+// converts those integers to float via irv_convert_to_float, so the caller
+// always fills line->i32 regardless of reversibility.
+TEST(CodestreamAPI, Compress128x128x4Ch_qstep0003)
+{
+  using param_nlt = ojph::param_nlt;
+  const ojph::ui32 width  = 128;
+  const ojph::ui32 height = 128;
+
+  ojph::codestream cs;
+
+  // QCD: quantization base step size.
+  cs.access_qcd().set_irrev_quant(0.0003f);
+
+  // SIZ: 128x128, components 0-2 are 8-bit unsigned, component 3 is 32-bit
+  // unsigned.
+  ojph::param_siz siz = cs.access_siz();
+  siz.set_image_extent(ojph::point(width, height));
+  siz.set_num_components(5);
+  for (ojph::ui32 c = 0; c < 3; ++c)
+    siz.set_component(c, ojph::point(1, 1), 8, false);
+  siz.set_component(3, ojph::point(1, 1), 32, true);
+  param_nlt nlt = cs.access_nlt();
+  nlt.set_nonlinear_transform(3, param_nlt::OJPH_NLT_BINARY_COMPLEMENT_NLT);
+  siz.set_component(4, ojph::point(1, 1), 32, false);
+
+  // COD: irreversible, no color transform (components differ in bit depth).
+  ojph::param_cod cod = cs.access_cod();
+  cod.set_reversible(false);
+  cod.set_color_transform(false);
+
+  // Planar mode: each component is pushed row-by-row in full before the next.
+  cs.set_planar(true);
+
+  ojph::mem_outfile outfile;
+  outfile.open();
+  cs.write_headers(&outfile);
+
+  ojph::ui32 next_comp;
+  ojph::line_buf *line = cs.exchange(nullptr, next_comp);
+
+  // Push all rows for all 4 components (planar order: comp 0 fully, then 1,
+  // then 2, then 3).
+  for (ojph::ui32 r = 0; r < 4 * height; ++r)
+  {
+    ojph::si32 *dp = line->i32;
+    if (next_comp < 3)
+    {
+      // 8-bit unsigned: values in [0, 255].
+      for (ojph::ui32 x = 0; x < width; ++x)
+        dp[x] = (ojph::si32)((r * 17u + x * 3u) & 0xFFu);
+    }
+    else
+    {
+      // 32-bit unsigned: keep values positive to stay within si32 range.
+      for (ojph::ui32 x = 0; x < width; ++x)
+        dp[x] = (ojph::si32)((r * 100000u + x * 1000u) & 0x7FFFFFFFu);
+    }
+    line = cs.exchange(line, next_comp);
+  }
+
+  cs.flush();
+  cs.close();
+
+  // Verify a non-empty J2C codestream was produced (SOC marker = 0xFF 0x4F).
+  EXPECT_GT(outfile.get_used_size(), 0u);
+  const ojph::ui8 *data = outfile.get_data();
+  ASSERT_NE(data, nullptr);
+  EXPECT_EQ(data[0], 0xFF);
+  EXPECT_EQ(data[1], 0x4F);
+}