Yes. Depending on your particular requirements, you may want to deviate but for flexibility create a slightly larger /boot partition (192M?, 256M?) and then use the rest as a single large PV for VG. Perform the RHEL4 install, name the LV(s) something easily identifying that they belong to RHEL4 install (R4LV00, R4LV01, ...). The swap partition/LV may be shared between them (I see no reason off-hand why not) so you can call it something like swapLV. Only use what you need so that you have ample space available on VG to perform RHEL5 install. Once RHEL4 install is complete, copy grub.conf for your reference and start RHEL5 install, select custom/manual partitioning, assign it the same /boot partition and do NOT format, using the same VG leave the existing ones alone, you may specify to use swapLV for RHEL5 swap, then create your RHEL5 LV(s) (R5LV00, R5LV01, ...) and proceed with installation. once install finishes and booted to RHEL5, make sure that RHEL4 files (kernels and initrds) are still in /boot and check grub.conf. if they are not in grub.conf then add the lines using the reference rhel4 grub.conf from earlier. That should allow you to select which one to boot from grub menu. I say this offers more flexibility by using a "shared" VG between the two instances because it allows allocation of any free space in the VG to either OSes' LV(s) if needed later depending on what is happening.
There are any number of possible ways to configure their dual-boot co-existence on bare-metal hardware. Another option may be to install RHEL5 and use RHEL5/Xen to create virtualized instance(s) of RHEL4 and/or RHEL5. Without knowing the reasons for your requirements/instances, not really sure what may be the best fit for your situation.
Hope this helps.