diff --git a/README.md b/README.md
index 03bf5b7..617ef42 100644
--- a/README.md
+++ b/README.md
@@ -80,13 +80,6 @@ with regard to installing or upgrading your OpenWRT router.
   OpenWRT upgrades often don't normalize upgraded config files in the same
   way from version to version
 
-## Raspberry Pi 4 Network Performance Tuning
-
-  * [(15) BIFRÖST Part 2 - RPi 4B Hybrid Bridging Router - Tuning Linux Network for Gigabit Line Speed | LinkedIn](https://www.linkedin.com/pulse/bifr%C3%B6st-rpi-4b-hybrid-routing-bridge-tuning-linux-network-corner/?trk=read_related_article-card_title)
-    very detailed article about tuning RPi4 network performance using IRQ
-    CPU pinning and other tricks. Kind of a hardwired ues of `irqbalance`
-
-
 ## Post Install Actions
 
 * [[OpenWrt Wiki] UCI defaults](https://openwrt.org/docs/guide-developer/uci-defaults)
@@ -125,6 +118,39 @@ with regard to installing or upgrading your OpenWRT router.
 
 Disable DHCP server on LAN: `uci set dhcp.lan.ignore=1 && uci commit && /etc/init.d/dnsmasq restart`
 
+# Raspberry Pi 4
+
+OpenWRT uses a single (main) partition for both the `/rom` and `rootfs_data`
+and uses a special `mtdsplit` driver to make this sub-partitioning.
+The `/rom` contains a squashfs filesystem with kernel and rootfs and
+`rootfs_data` is where the writable `overlayfs` is mounted:
+[[OpenWrt Wiki] The OpenWrt Flash Layout](https://openwrt.org/docs/techref/flash.layout)
+
+For some reason, on a Raspberry Pi 4, this partition is by default only 104MB.
+This leaves only around 70MB writable space. Run-time re-partitioning is quite
+complicated (contact me if you know how to do it) so that leaves us with
+the only option of building the image with a pre-determined size.
+The `oi-build` script takes the value of `CONFIG_TARGET_ROOTFS_PARTSIZE` as
+the size of the main partition. Make sure that you don't make it larger than
+the SD-card you are using.
+
+NOTE: Contrary to what the OpenWRT Wiki says,
+`CONFIG_TARGET_ROOTFS_PARTSIZE` is ignored by the "make" command-line,
+and you have to set it in the `.config` file instead.
+
+See additional info here from the forum:
+  * [Building a x86-based image and need more disk space in the image - For Developers - OpenWrt Forum](https://forum.openwrt.org/t/building-a-x86-based-image-and-need-more-disk-space-in-the-image/2585/5)
+  * [X86 sysupgrade not working - For Developers - OpenWrt Forum](https://forum.openwrt.org/t/x86-sysupgrade-not-working/37869)
+
+https://forum.openwrt.org/t/x86-sysupgrade-not-working/37869
+
+## Raspberry Pi 4 Network Performance Tuning
+
+  * [(15) BIFRÖST Part 2 - RPi 4B Hybrid Bridging Router - Tuning Linux Network for Gigabit Line Speed | LinkedIn](https://www.linkedin.com/pulse/bifr%C3%B6st-rpi-4b-hybrid-routing-bridge-tuning-linux-network-corner/?trk=read_related_article-card_title)
+    very detailed article about tuning RPi4 network performance using IRQ
+    CPU pinning and other tricks. Kind of a hardwired ues of `irqbalance`
+
+
 # Similar projects
 
   * [Github ansemjo/openwrtbuilder](https://gist.github.com/ansemjo/cb41677a76f1c063527744438b03b932)
diff --git a/configs/rpi-4 b/configs/rpi-4
index 02271a5..6489aef 100644
--- a/configs/rpi-4
+++ b/configs/rpi-4
@@ -10,6 +10,13 @@ PROFILE="rpi-4"
 
 RELEASE=${RELEASE_21_02}
 
+# Define this if you want a larger file writable system
+# The default for RPi4 is 104 (MB)
+# Here we set it to 4096 (MB) which is almost 4GB.
+# You must lower the value if your SD-card is smaller than that.
+CONFIG_TARGET_ROOTFS_PARTSIZE=4096
+
+
 # See https://openwrt.org/packages/start
 #
 # - irqbalance package distributes hardware interrupts across
diff --git a/oi-build b/oi-build
index f47bd16..1c6a588 100755
--- a/oi-build
+++ b/oi-build
@@ -138,6 +138,14 @@ image_build() {
 
     locate_files_dir
 
+    if [ -n "${CONFIG_TARGET_ROOTFS_PARTSIZE}" ]; then
+        local configfile="${imagebuilder_dir}/.config"
+        echo_note "Configuring CONFIG_TARGET_ROOTFS_PARTSIZE=${CONFIG_TARGET_ROOTFS_PARTSIZE}"
+        echo_note "by appending to ${configfile}"
+        echo "CONFIG_TARGET_ROOTFS_PARTSIZE=${CONFIG_TARGET_ROOTFS_PARTSIZE}" \
+            >> .config
+    fi
+
     PACKAGES=$(echo "${PACKAGES}" | tr '\n' ' ')
     echo_run "make image PROFILE='${PROFILE}' FILES='${FILES}' PACKAGES='${PACKAGES}' DISABLED_SERVICES='${DISABLED_SERVICES}'" ||
         die "image build FAILED"
@@ -299,4 +307,13 @@ fi
 if [ -d bin/targets/${TARGET} ]; then
     echo_run "ls -lh bin/targets/${TARGET}/"
     echo_note "Final images are in ${imagebuilder_dir}/bin/targets/${TARGET}"
+    cat << EOF
+Run following commands to perform a system upgrade:
+  scp FILE.img.gz HOST:/tmp
+  ssh HOST '/sbin/sysupgrade -v /tmp/*.img.gz'
+
+For example:
+  scp openwrt-imagebuilder-21.02.0-rc3-bcm27xx-bcm2711.Linux-x86_64/bin/targets/bcm27xx/bcm2711/openwrt-21.02.0-rc3-bcm27xx-bcm2711-rpi-4-ext4-sysupgrade.img.gz 192.168.1.1:/tmp
+  ssh 192.168.1.1 '/sbin/sysupgrade -v /tmp/*.img.gz'
+EOF
 fi