I need to install a shower and toilet on a concrete floor the home 4" drain is 4 feet away and can be tapped into. What's the best way to build up a platform and how high does it need to be to make it work

I would use at least 2x10, or even 2x12 if you have enough head room. By the time you get a 4x4x3 tee fitting into ci stack and use a 3" 90 for toilet bend, with proper slope. You may make it with 2x8s, but it will be aweful close. Just make you platform from the ?x4s. Stand them up and place them on 16" centers. Make sure to run the studs parallel to toilet drain, so you won't have to cut any large holes in studs to run drain. You will most likely have to drill holes for shower drain, but that's OK.

I would use at least 2x10, or even 2x12 if you have enough head room. By the time you get a 4x4x3 tee fitting into ci stack and use a 3" 90 for toilet bend, with proper slope. You may make it with 2x8s, but it will be aweful close. Just make you platform from the ?x4s. Stand them up and place them on 16" centers. Make sure to run the studs parallel to toilet drain, so you wont have to cut any large holes in studs to run drain. You will most likely have to drill holes for shower drain, but thats ok.
Should I use a trap for the shower drain

MGD gave you some good pointers on platforming your fixtures but I don't see any mention of vents. Why cobble the job? Bite the bullet, take up the slab and do it like a plumber would. And since you're going to hafta vent the shower anyhow why not add a lavatory/vanity and wet vent it through the lavatory vent. Most bathroom groups are roughed in like this.
Toilet connects to sewer main or the stack vent. Lavatory connects to toilet drain and runs a vent off the top the stubout tee out the roof or revents back into a dry vent in the attic.. The toilet wet vents through the lavatory vent and the tub/shower connects to the lavatory drain and is wet vented by it. This is a normal rough in and is acceptable both by local and state codes and also The Standard Plumbing Code Book in 90 percent of the country. Check your local codes to make sure you're not in the excluded 10 percent.. The vent off the lavatory may be run out the roof or revented back into a dry vent in the attic or if you're reventing back into a fixtures dry vent you must make your connection at least 6 inches over that fixtures flood rim. Good luck and thank you for rating my answer. Tom

Yes, you definitely need a trap for shower drain. You also must vent toilet and shower.

You can still do this without tearing out concrete(by buiding your platform), but you will need to find an area or build a stub wall to take vent up and tie in as per Toms instructions.