diff --git a/quic/core/congestion_control/bbr2_misc.h b/quic/core/congestion_control/bbr2_misc.h
index fb66b98..a603064 100644
--- a/quic/core/congestion_control/bbr2_misc.h
+++ b/quic/core/congestion_control/bbr2_misc.h
@@ -9,6 +9,7 @@
 #include <limits>
 
 #include "quic/core/congestion_control/bandwidth_sampler.h"
+#include "quic/core/congestion_control/send_algorithm_interface.h"
 #include "quic/core/congestion_control/windowed_filter.h"
 #include "quic/core/quic_bandwidth.h"
 #include "quic/core/quic_packet_number.h"
@@ -426,6 +427,10 @@
     bandwidth_sampler_.SetLimitMaxAckHeightTrackerBySendRate(value);
   }
 
+  void SetMaxAckHeightTrackerWindowLength(QuicRoundTripCount value) {
+    bandwidth_sampler_.SetMaxAckHeightTrackerWindowLength(value);
+  }
+
   bool MaybeExpireMinRtt(const Bbr2CongestionEvent& congestion_event);
 
   QuicBandwidth BandwidthEstimate() const {
diff --git a/quic/core/congestion_control/bbr2_sender.cc b/quic/core/congestion_control/bbr2_sender.cc
index f1c96d6..6938f6d 100644
--- a/quic/core/congestion_control/bbr2_sender.cc
+++ b/quic/core/congestion_control/bbr2_sender.cc
@@ -120,6 +120,16 @@
 
 void Bbr2Sender::ApplyConnectionOptions(
     const QuicTagVector& connection_options) {
+  if (GetQuicReloadableFlag(quic_bbr2_extra_acked_window) &&
+      ContainsQuicTag(connection_options, kBBR4)) {
+    QUIC_RELOADABLE_FLAG_COUNT_N(quic_bbr2_extra_acked_window, 1, 2);
+    model_.SetMaxAckHeightTrackerWindowLength(20);
+  }
+  if (GetQuicReloadableFlag(quic_bbr2_extra_acked_window) &&
+      ContainsQuicTag(connection_options, kBBR5)) {
+    QUIC_RELOADABLE_FLAG_COUNT_N(quic_bbr2_extra_acked_window, 2, 2);
+    model_.SetMaxAckHeightTrackerWindowLength(40);
+  }
   if (ContainsQuicTag(connection_options, kBBQ2)) {
     params_.startup_cwnd_gain = 2.885;
     params_.drain_cwnd_gain = 2.885;
diff --git a/quic/core/congestion_control/bbr2_simulator_test.cc b/quic/core/congestion_control/bbr2_simulator_test.cc
index 3b08905..9c2cca1 100644
--- a/quic/core/congestion_control/bbr2_simulator_test.cc
+++ b/quic/core/congestion_control/bbr2_simulator_test.cc
@@ -483,6 +483,46 @@
   EXPECT_APPROX_EQ(params.RTT(), rtt_stats()->min_rtt(), 0.2f);
 }
 
+TEST_F(Bbr2DefaultTopologyTest, SimpleTransferBBR4) {
+  SetQuicReloadableFlag(quic_bbr2_extra_acked_window, true);
+  SetConnectionOption(kBBR4);
+  DefaultTopologyParams params;
+  CreateNetwork(params);
+
+  // Transfer 12MB.
+  DoSimpleTransfer(12 * 1024 * 1024, QuicTime::Delta::FromSeconds(35));
+  EXPECT_TRUE(Bbr2ModeIsOneOf({Bbr2Mode::PROBE_BW, Bbr2Mode::PROBE_RTT}));
+
+  EXPECT_APPROX_EQ(params.BottleneckBandwidth(),
+                   sender_->ExportDebugState().bandwidth_hi, 0.01f);
+
+  EXPECT_LE(sender_loss_rate_in_packets(), 0.05);
+  // The margin here is high, because the aggregation greatly increases
+  // smoothed rtt.
+  EXPECT_GE(params.RTT() * 4, rtt_stats()->smoothed_rtt());
+  EXPECT_APPROX_EQ(params.RTT(), rtt_stats()->min_rtt(), 0.2f);
+}
+
+TEST_F(Bbr2DefaultTopologyTest, SimpleTransferBBR5) {
+  SetQuicReloadableFlag(quic_bbr2_extra_acked_window, true);
+  SetConnectionOption(kBBR5);
+  DefaultTopologyParams params;
+  CreateNetwork(params);
+
+  // Transfer 12MB.
+  DoSimpleTransfer(12 * 1024 * 1024, QuicTime::Delta::FromSeconds(35));
+  EXPECT_TRUE(Bbr2ModeIsOneOf({Bbr2Mode::PROBE_BW, Bbr2Mode::PROBE_RTT}));
+
+  EXPECT_APPROX_EQ(params.BottleneckBandwidth(),
+                   sender_->ExportDebugState().bandwidth_hi, 0.01f);
+
+  EXPECT_LE(sender_loss_rate_in_packets(), 0.05);
+  // The margin here is high, because the aggregation greatly increases
+  // smoothed rtt.
+  EXPECT_GE(params.RTT() * 4, rtt_stats()->smoothed_rtt());
+  EXPECT_APPROX_EQ(params.RTT(), rtt_stats()->min_rtt(), 0.2f);
+}
+
 TEST_F(Bbr2DefaultTopologyTest, SimpleTransferSmallBuffer) {
   DefaultTopologyParams params;
   params.switch_queue_capacity_in_bdp = 0.5;
diff --git a/quic/core/quic_flags_list.h b/quic/core/quic_flags_list.h
index 727f1e2..5773ec5 100644
--- a/quic/core/quic_flags_list.h
+++ b/quic/core/quic_flags_list.h
@@ -135,6 +135,8 @@
 QUIC_FLAG(FLAGS_quic_reloadable_flag_quic_bbr2_ignore_inflight_hi_in_probe_up, true)
 // When true, the BBQ0 connection option causes QUIC BBR2 to add bytes_acked to probe_up_acked if the connection hasn\'t been app-limited since inflight_hi was utilized.
 QUIC_FLAG(FLAGS_quic_reloadable_flag_quic_bbr2_add_bytes_acked_after_inflight_hi_limited, true)
+// When true, the BBR4 copt sets the extra_acked window to 20 RTTs and BBR5 sets it to 40 RTTs.
+QUIC_FLAG(FLAGS_quic_reloadable_flag_quic_bbr2_extra_acked_window, true)
 
 #endif