Well, the first thing to check out is whether you like the Phase 90 tone. It's not an exaggeration to say that likely 2/3 of the phasers on the market use that circuit. If you do, and you use active basses with a hot output you'll need to be careful as the "script circuit" can be prone to clipping/distorting. Even with passive basses it can happen with aggressive playing. As RCCollins pointed out, there are some that are much better about this than others.
The other major 4 stage phaser is the EHX Small Stone which has a darker character to it. There's a very hot rodded version from Blackout Effectors called the Whetstone. There are many cool 4 stage phasers such as the Subdecay Baby Quasar, HBE Psilocybe, Roland AP-2 and others. If you want something more thick and "pulse-like" you can try an old DOD 490 or Earthquaker Grand Orbiter.
If you like the Phase 90 but want something more subtle, there's always the Phase 45 (a two stage phaser) or it's derivatives like the DOD 201, lovepedal vibe or Tone Factor Nebula.
Of course there's also six stage phasers (DOD 595, Keeley), 8 stage phasers (Jet Phaser), 10 stage phasers (MXR Phase 100, Maxon PT-9) and ones that can switch the number of stages (Moog, new Subdecay Quasar Deluxe, TC Electronic, Maxon PH-350)
Not to mention Univibe pedals and envelope phasers like the Pigtronix, Polyphase and Phantasm (which has some Phase 90 character to it)
And one more layer of complexity there are also some digital phasers (Digitech, Boss, Source Audio - I think, EBS and others) if you want to go that route.
It's impossible to say what will sound best to you. Personally I like shimmery 6 stage phasers that have a prominent sound in the mix and work well with an envelope filter. So I've ended up with the Keeley Phaser.
But others might one with a slow sweep that sounds good with dirt pedals or a subtle, watery tone, a high stage and electronica sounding phaser, a classic tone (Thin Lizzy, Stranglehold, For the Love of Money),a funky and rubbery envelope phaser or a completely crazy sound making machine like the Deluxe Quasar or Infinitphase.
It really depends on what you want. FWIW, my stock answer (for my prefereces) is usually to recommend the EHX Nano Small Stone. It's small, cheap and (IMO) sounds great. But if you have more specific preferences I could give a more specific recommendation.
Hope that helps.