Setting access point on Raspberry Pi 4 running Ubuntu 18.04 bionic

This scenario will bridge wlan0 and eth0 interfaces. eth0 is connected to Internet and wlan0 will be used by the access point.

Disable network manager!

 apt install ifupdown
 systemctl unmask networking
 systemctl enable networking
 systemctl restart networking
 systemctl stop systemd-networkd.socket systemd-networkd networkd-dispatcher systemd-networkd-wait-online
 systemctl disable systemd-networkd.socket systemd-networkd networkd-dispatcher systemd-networkd-wait-online
 systemctl mask systemd-networkd.socket systemd-networkd networkd-dispatcher systemd-networkd-wait-online
 apt purge nplan netplan.io

Install and configure hostapd

 apt install hostapd
 vi /etc/hostapd/hostapd.conf
 interface=wlan0
 bridge=br0

 #SSID to be used in IEEE 802.11 management frames
 ssid=<ssid>
 wpa_passphrase=<password>
 
 # Disable broadcasting SSID
 #ignore_broadcast_ssid=1
 
 # Driver interface type (hostap/wired/none/nl80211/bsd)
 #driver=nl80211
 
 # Country code (ISO/IEC 3166-1)
 #country_code=US

 # Operation mode (a = IEEE 802.11a (5 GHz), b = IEEE 802.11b (2.4 GHz)
 #hw_mode=g

 # Channel number
 channel=1

 # Maximum number of stations allowed
 #max_num_sta=5

 # Bit field: bit0 = WPA, bit1 = WPA2
 wpa=2

 # Bit field: 1=wpa, 2=wep, 3=both
 auth_algs=1

 # Set of accepted cipher suites; disabling insecure TKIP
 wpa_pairwise=CCMP

 # Set of accepted key management algorithms
 wpa_key_mgmt=WPA-PSK

 # hostapd event logger configuration
 logger_stdout=-1
 logger_stdout_level=3

 # Uncomment and modify the following section if your device supports 802.11n
 ## Enable 802.11n support
 #ieee80211n=1
 ## QoS support
 #wmm_enabled=1
 ## Use "iw list" to show device capabilities and modify ht_capab accordingly
 #ht_capab=[HT20/HT40][Static SM Power Save][RX HT20 SGI][RX HT40 SGI][No RX STBC][Max AMSDU length: 3839 bytes][DSSS/CCK HT40]

 vi /lib/systemd/system/hostapd.service
 [Unit]
 Description=Advanced IEEE 802.11 AP and IEEE 802.1X/WPA/WPA2/EAP Authenticator
 After=network.target

 [Service]
 Type=forking
 PIDFile=/run/hostapd.pid
 EnvironmentFile=/etc/default/hostapd
 ExecStart=/usr/sbin/hostapd -P /run/hostapd.pid -B $DAEMON_OPTS ${DAEMON_CONF}
 ExecStartPost=/sbin/iw wlan0 set txpower limit 100

 [Install]
 WantedBy=multi-user.target

 sudo systemctl unmask hostapd
 sudo systemctl enable hostapd
 

Install and configure bridge-utils

 apt install bridge-utils
 vi /etc/network/interfaces
 auto lo
 iface lo inet loopback
 auto br0
 #iface wlan0 wireless-txpower 1
 iface br0 inet static
        address <ip-addr>
        network <net-addr>
        netmask <net-mask>
        broadcast <brdcst-ip>
        gateway <gw-ip>
        bridge_ports eth0
        bridge_stp off

Add br_netfilter module for iptables to see the bridge traffic

 vi /etc/modules-load.d/bridge.conf
 br_netfilter

Enable IPv4 forwarding and optionally disable IPv6

 vi /etc/sysctl.conf
 net.ipv6.conf.all.disable_ipv6 = 1
 net.ipv6.conf.default.disable_ipv6 = 1
 net.ipv6.conf.lo.disable_ipv6 = 1
 net.ipv6.conf.eth0.disable_ipv6 = 1
 net.ipv6.conf.wlan0.disable_ipv6 = 1
 net.ipv4.ip_forward = 1

Restart

 reboot