I tried to boot with the latest (2b76cfc) start4.elf and fixup4.dat files on Pi4 (8GB), but instead I got seven ACT LED flashes (kernel.img not found). The boot is OK with the previous commit 9d4f640.

Here is the log:
PM_RSTS: 0x00001000
RPi: BOOTLOADER release VERSION:c302dea0 DATE: Jun 15 2020 TIME: 14:36:22 BOOTMODE: 0x00000006 part: 0 BUILD_TIMESTAMP=1592228179 0x289c4502 0x00d03114
uSD voltage 3.3V
Initialising SDRAM 'Micron' 32Gb x2 total-size: 64 Gbit 3200
Boot mode: USB-MSD (04) order f1
VLI: HUB2: 0xfff00000 0x24e6 MCU: 0xfff20000 0x150d8
VL805 0xfff00000 0xfff20000
XHCI-STOP
xHC ver: 256 HCS: 05000420 fc000031 00e70004 HCC: 002841eb
xHC ports 5 slots 32 intrs 4
USB3 rport 2 status 00000000 -> 000002a0
USB3 rport 3 status 00000000 -> 000002a0
USB3 rport 4 status 00000000 -> 000002a0
USB3 rport 5 status 00000000 -> 000002a0
USB2 rport 1 status 00000000 -> 400002e1
USB2 rport 1 status 400002f1 -> 40200e03
root HUB port 1 init
DEV [01:00] 2.16 000000:01 class 9 VID 2109 PID 3431
HUB init [01:00] 2.16 000000:01
USB3 rport 3 status 000002a0 -> 00021203
root HUB port 3 init
DEV [02:00] 3.16 000000:03 class 0 VID 174c PID 55aa
MSD device [02:00] 3.16 000000:03 conf 0 iface 0 ep 81#1024 02#1024
MSD [02:00] 3.16 000000:03 register MSD
PM_RSTS: 0x00001000
MSD [02:00] 3.16 000000:03 LUN 0
MSD [02:00] 3.16 000000:03 lun 0 block-count 3907029168 block-size 512
MBR: 0x00000001,3907029167 type: 0xee
MBR: 0x00000000, 0 type: 0x00
MBR: 0x00000000, 0 type: 0x00
MBR: 0x00000000, 0 type: 0x00
GPT dcc3cf174503604e9ac54251ae442ebd 000000001 0e8e088af num-partitions 128 entry-size 128
GPT partition: 0 a2a0d0ebe5b9334487c068b6b72699c7 000000800..0000807ff
lba: 2048 oem: 'mkfs.fat' volume: ' boot '
rsc 32 fat-sectors 4033 c-count 516190 c-size 1 r-dir 2 r-sec 0
PM_RSTS: 0x00001000
Partition: 0
GPT dcc3cf174503604e9ac54251ae442ebd 000000001 0e8e088af num-partitions 128 entry-size 128
GPT partition: 0 a2a0d0ebe5b9334487c068b6b72699c7 000000800..0000807ff
lba: 2048 oem: 'mkfs.fat' volume: ' boot '
rsc 32 fat-sectors 4033 c-count 516190 c-size 1 r-dir 2 r-sec 0
Read config.txt bytes 475 hnd 0x000529f0 hash '57de04aefcb7f6b1'
recover4.elf not found (6)
recovery.elf not found (6)
Read start4.elf bytes 2277216 hnd 0x0005188e hash 'c4f805aae1aba767'
Read fixup4.dat bytes 5409 hnd 0x00051883 hash 'd9b8d1561115150d'
0x00d03114 0x00000000 0x0000001f
MEM GPU: 76 ARM: 947 TOTAL: 1023
Starting start4.elf @ 0xfeb00200 partition 0

MESS:00:00:07.723545:0: bfs_xhci_retrieve 3ee76220
MESS:00:00:08.252170:0: gpioman: gpioman_get_pin_num: pin DISPLAY_DSI_PORT not defined
MESS:00:00:08.259419:0: *** Restart logging
MESS:00:00:08.264301:0: hdmi: HDMI:hdmi_get_state is deprecated, use hdmi_get_display_state instead
MESS:00:00:08.273599:0: hdmi: HDMI:hdmi_get_state is deprecated, use hdmi_get_display_state instead
MESS:00:00:08.279532:0: HDMI0: hdmi_pixel_encoding: 300000000

MESS:00:00:08.285086:0: HDMI1: hdmi_pixel_encoding: 300000000

MESS:00:00:08.295353:0: dtb_file 'bcm2711-rpi-4-b.dtb'
MESS:00:00:08.297414:0: Failed to load Device Tree file '?'
MESS:00:00:08.302707:0: Failed to open command line file 'cmdline.txt'
MESS:00:00:08.309589:0: No compatible kernel found