As Peter has said, the oblique barre, which is properly used to indicate alternatives, is very commonly used to indicate "as well as", or a span or continuation. This is the case here.

The four phases of clinical trials (or five according to more recent definitions) are well defined. Phase 3 is "testing of drug for intended use as therapy", and Phase 4 is "postmarketing surveillance - watching drug use in public".
http://en.wikipedia.org/wiki/Phases_of_clinical_research
There is no actual intermediate phase (though the second part of Phase 3 is often called 3b (or IIIb). However, there are trials which extend over more than one phase, and this is the case here:

"Clinical trials are classified by phase most commonly for drugs, biologics, and devices in the FDA approval process for marketing (although the trials themselves may extend beyond the date of approval). NIH also describes trials by phase, for example, trials in cancer and AIDS, which are listed in their respective registries by phase. Clear lines do not necessarily demarcate one phase from another, however, and trials may be denoted as "phase 1/2" or "phase 3/4.""
http://www.nap.edu/openbook.php?record_id=9742&page=69

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Note added at 10 hrs (2012-08-11 20:28:22 GMT)
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This does not mean Phase 3 out of 4. There would be no point in adding "/4" for "out of 4" if it referred only to Phase 3, because everyone involved in clinical trials knows that there are four phases. And clearly "Phase 1/2" doesn't mean 1 out of 2, because there are never just two phases; it means phases 1 and 2. "Phase 3/4" definitely means trials corresponding to Phases 3 and 4. It is common usage in that sense:

" If the Phase 2 trials show that the drug possibly works, then the Phase 3/4 trials begin. Phase 3/4 trials are VERY large trials conducted over a VERY long period of time. In fact, many times the Phase 3/4 trials will happen even after a drug has been marketed to the public already. (There are still some Viagara Phase 3 and Phase 4 trials taking place). In these trials the goal is to see if there are any long-term side effects of taking the drug. Sample sizes in excess of 1,000+ patients are no uncommon and the trial lengths can exceed 10 years. "
http://www.retrojunk.com/forum/post/list/7542/2

It is also sometimes written as "Phase 3,4" or "Phase 3-4":

"Interventional clinical trials, usually drug trials, are categorised into different phases. A phase 1 trial is an initial study to determine side effects and the maximum tolerated dose. A phase 2 trial includes trials that test the effectiveness of a drug as well as side effects. Phase 3 trials are usually randomised and test the new treatment against the standard treatment. Phase 4 trials are post-marketing studies to monitor side effects, benefits and optimal use of an intervention in the wider population. A category “not applicable” is available for non-drug trials. Phase is not compulsory for registrants on the ANZCTR so “not recorded” was entered for trials with blanks. For the CT.gov trials, phase is a compulsory data item. For trials classified as Phase 1/2 these were recoded as Phase 2, Phase 2/3 trials were recoded as Phase 3 trials One ANZCTR trial registered as Phase 3/4 trials was recoded as a Phase 4 trial as this best fit the study design. "
http://www.psych.usyd.edu.au/cemped/docs/Appendix_revised.pd...


"Phase 3/4 clinical trial data is, by far, the best quality, albeit with much lower sample size. [...] Prospective, randomized, double-blind clinical trials (Phase 3 and 4) provide by far the best quality data for assessment of drug effects on adverse events."
http://www.dddmag.com/articles/2008/04/digging-data
"Where relevant, has adequate justification been provided for
hybrid study design (e.g. Phase 1/2 or Phase 3/4 studies)?"
http://www.fpnsw.org.au/scientific_advisory_group_tor_final....

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Note added at 23 hrs (2012-08-12 09:20:28 GMT)
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It is not true that a clinical trial is necessarily confined to a single phase. On the contrary, trials frequently correspond to more than one phase:

"Four phases of clinical trials and medicine development exist and are defined below. Each of these definitions is a functional one and the terms are not defined on a strict chronological basis. An investigational medicine is often evaluated in two or more phases simultaneously in different clinical trials. Also, some clinical trials may overlap two different phases."
http://www.virginia.edu/vpr/irb/HSR_docs/CLINICAL_TRIALS_Pha...

That is what "Phase 1/2" or "Phase 3/4" means, or indeed "Phase 3-4":

"Sulodexide Gelcaps : An Oral Heparinoid for the Treatment of Diabetic Nephropathy Gets Ready for Phase 3-4 Trial [...]
Keryx Biopharmaceuticals, Inc. announced today that it has finalized a Special Protocol Assessment (SPA) agreement with the Food and Drug Administration (FDA) for the Phase 3 and Phase 4 clinical trials of KRX-101 (sulodexide gelcaps), the Company's drug candidate for the treatment of diabetic nephropathy."
http://www.rxpgnews.com/nephrology/Sulodexide_Gelcaps_An_Ora...