I am adding this as an answer as I don't yet have 15 rep to post a comment, but this is no more than an extension of Martin's answer.
Stay away from shortcutting numbers in domain / company names. Seriously. As far away as you can.
I have recently started looking into the process of getting a resident's visa to the UK. As it turns out, the UK government does not handle Visa applications themselves. Instead, they do so via an outsourced company based in NY, US. The company's website is
Does that sound remotely serious to you ?
The first thought that crossed my mind was "This has to be some sort of scam". I know it is a very personal statement, but it's all about public perception. In my particular case (and opinion), something along the lines of
Or something along those lines would've given me FAR more comfort, at least initially (that point at which you are still not sure if you are dealing with a serious business or not).
Getting both domain names sounds like a great idea, to avoid any complications with third parties in the future. But please, please, do not shortcut words with numbers. At least if your goal is to establish yourself as a serious entrepreneur.
Of course, everything said is completely subjective.
Have a look at the search volume in the Adwords Keyword Tool.
Make sure to use "exact match ".
This will tell you what people really search.
Do they type "for" or "4".
Here are some examples of search volume from the tool:
[body for life] 12,100 / month
[body 4 life] 170 / month
For SEO reasons you can see in this example it's much better to have "for". That's what people type in; that is what they will wish to see.
Now, I do not know what domain you are considering. But I advise you to drill down the Adwords Keyword tool data and get the domain name as exact match (no hyphen, no number, no trick) and DOTCOM. If you cannot afford it, then the keyword market is probably too competitive for you; find a niche that has some search volume : go down in the search volumes and make sure you get a perfact match dotcom.
if you sell shoes, get shoes.com if you cannot afford it, drill down find next top searched "shoes" keyword, say cool shoes, then see if you can afford coolshoes.com, etc.
Way to many people tend to think that choosing the right domain is very important, but the fact is that it doesn't really matter.
If your product is good people will find it no matter how obscure your domain might sound.
Should you get a .net domain? No most people will say a .com domain is much better.
Yet a company like Pandora.net has risen to incredible success because people want their products.
But they are a jewelry company you might say. And to that all I can say is that their iPhone app was ranking 75 last week.
Successful names are successful in retrospect.
Don't get to hung up on doing one thing or another. What feels best to you, that is the one you should choose.
LOL, go search for the domains first before you get too attached. Both of those domains are unavailable (I knew they would be just from the keywords).
If you are starting out and have not branded your company yet with either of those names, I strongly recommend going to a domain seller's site and using their tools to search for available domains. No sense falling in love with a name if you don't even know if you can register it or buy it from the current owner.
As for which of those 2 names is better, it depends on your market. I can see a youth-oriented health site being better as body4life (yes, you have to shift on mobile for a number, but texters can do that without blinking and shift+4 or pressing an extra second until it changes to a 4 is really not a big concern relative to having a memorable and easy to spell name that matches your brand image).
If your brand targets adults, especially college-educated adults, you might want to avoid replacing words with numbers as that annoys a lot of grammar nazis and is negatively associated with l33t speak and texting kids. Like other commenters mentioned, this makes a bad first impression if your site is supposed to be a serious, authoritative resource for something important like health, finance, career advice, etc. but for a teen or young adult audience a number might seem more accessible/hip (shorter = good too).
If you aren't sure which is better, find people in your target market and ask them which name they prefer.
Don't be discouraged that your first name ideas were taken, I've spent many hours searching for a domain name for one site because everything I can think of is already taken. That's just how it is with combinations of common and popular English words. Health + Body are two really popular keywords for affiliate marketers and spammers so there are few 2-3 word combinations with those terms now. Also, if you find a domain and it is already owned, don't be shy about emailing an offer for it. You can also place a backorder which is paying for nothing today, but if the domain ever becomes available, the service will allow you to grab it fairly quickly (I've done this successfully, but it took 2-3 years for the domain I wanted to come back on the market).